From cae926a2b04a4dff710b1cf4622b81c08fabfe46 Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Tue, 10 Dec 2024 17:30:53 +0100
Subject: [PATCH 01/16] WIP: enable dragging entities from the

---
 crates/viewer/re_time_panel/src/lib.rs        |   3 +-
 crates/viewer/re_ui/src/design_tokens.rs      |  11 ++
 .../viewer/re_ui/src/list_item/list_item.rs   |   2 +-
 .../viewer/re_viewer_context/src/contents.rs  |  10 +-
 .../re_viewer_context/src/drag_and_drop.rs    | 185 +++++++++++-------
 crates/viewer/re_viewport/src/viewport_ui.rs  |  71 ++++++-
 6 files changed, 188 insertions(+), 94 deletions(-)

diff --git a/crates/viewer/re_time_panel/src/lib.rs b/crates/viewer/re_time_panel/src/lib.rs
index 39e67101dc4d..dc04da490d8e 100644
--- a/crates/viewer/re_time_panel/src/lib.rs
+++ b/crates/viewer/re_time_panel/src/lib.rs
@@ -673,6 +673,7 @@ impl TimePanel {
         } = ui
             .list_item()
             .selected(is_selected)
+            .draggable(true)
             .force_hovered(is_item_hovered)
             .show_hierarchical_with_children(
                 ui,
@@ -726,7 +727,7 @@ impl TimePanel {
             &response,
             SelectionUpdateBehavior::UseSelection,
         );
-        ctx.handle_select_hover_drag_interactions(&response, item.to_item(), false);
+        ctx.handle_select_hover_drag_interactions(&response, item.to_item(), true);
 
         let is_closed = body_response.is_none();
         let response_rect = response.rect;
diff --git a/crates/viewer/re_ui/src/design_tokens.rs b/crates/viewer/re_ui/src/design_tokens.rs
index 50acbbd2e7e4..8d6a6f8a53f3 100644
--- a/crates/viewer/re_ui/src/design_tokens.rs
+++ b/crates/viewer/re_ui/src/design_tokens.rs
@@ -430,6 +430,17 @@ impl DesignTokens {
     pub fn thumbnail_background_color(&self) -> egui::Color32 {
         self.color(ColorToken::gray(S250))
     }
+
+    /// Stroke used to indicate that a UI element is a container that will receive a drag-and-drop
+    /// payload.
+    ///
+    /// Sometimes this is the UI element that is being dragged over (e.g., a view receiving a new
+    /// entity). Sometimes this is a UI element not under the pointer, but whose content is
+    /// being hovered (e.g., a container in the blueprint tree)
+    #[inline]
+    pub fn drop_target_container_stroke(&self) -> egui::Stroke {
+        egui::Stroke::new(1.0, self.color(ColorToken::blue(S350)))
+    }
 }
 
 // ----------------------------------------------------------------------------
diff --git a/crates/viewer/re_ui/src/list_item/list_item.rs b/crates/viewer/re_ui/src/list_item/list_item.rs
index cdee8a6b3f06..275505a00659 100644
--- a/crates/viewer/re_ui/src/list_item/list_item.rs
+++ b/crates/viewer/re_ui/src/list_item/list_item.rs
@@ -389,7 +389,7 @@ impl ListItem {
                     Shape::rect_stroke(
                         bg_rect_to_paint.expand(-1.0),
                         0.0,
-                        egui::Stroke::new(1.0, ui.visuals().selection.bg_fill),
+                        crate::design_tokens().drop_target_container_stroke(),
                     ),
                 );
             }
diff --git a/crates/viewer/re_viewer_context/src/contents.rs b/crates/viewer/re_viewer_context/src/contents.rs
index 38149f711a25..558fbe8ad941 100644
--- a/crates/viewer/re_viewer_context/src/contents.rs
+++ b/crates/viewer/re_viewer_context/src/contents.rs
@@ -5,7 +5,7 @@ use egui_tiles::TileId;
 use re_log_types::EntityPath;
 
 use crate::item::Item;
-use crate::{BlueprintId, BlueprintIdRegistry, ContainerId, ItemCollection, ViewId};
+use crate::{BlueprintId, BlueprintIdRegistry, ContainerId, ViewId};
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Contents {
@@ -65,14 +65,6 @@ impl Contents {
     }
 }
 
-impl TryFrom<&ItemCollection> for Vec<Contents> {
-    type Error = ();
-
-    fn try_from(value: &ItemCollection) -> Result<Self, Self::Error> {
-        value.iter().map(|(item, _)| item.try_into()).collect()
-    }
-}
-
 impl TryFrom<Item> for Contents {
     type Error = ();
 
diff --git a/crates/viewer/re_viewer_context/src/drag_and_drop.rs b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
index bec7e7e43a5f..63bc95fff5bc 100644
--- a/crates/viewer/re_viewer_context/src/drag_and_drop.rs
+++ b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
@@ -1,10 +1,11 @@
 //! Implement a global drag-and-drop payload type that enable dragging from various parts of the UI
 //! (e.g., from the streams tree to the viewport, etc.).
 
-use std::fmt::Formatter;
+use std::fmt::{Display, Formatter};
 
 use itertools::Itertools;
-
+use re_entity_db::InstancePath;
+use re_log_types::EntityPath;
 use re_ui::{
     ColorToken, Hue,
     Scale::{S325, S375},
@@ -19,35 +20,63 @@ pub enum DragAndDropPayload {
     /// The dragged content is made only of [`Contents`].
     Contents { contents: Vec<Contents> },
 
+    /// The dragged content is made of entities.
+    Entities { entities: Vec<EntityPath> },
+
     /// The dragged content is made of a collection of [`Item`]s we do know how to handle.
     Invalid,
 }
 
 impl DragAndDropPayload {
     pub fn from_items(selected_items: &ItemCollection) -> Self {
-        if let Ok(contents) = selected_items.try_into() {
+        if let Some(contents) = try_item_collection_to_contents(selected_items) {
             Self::Contents { contents }
+        } else if let Some(entities) = try_item_collection_to_entities(selected_items) {
+            Self::Entities { entities }
         } else {
             Self::Invalid
         }
     }
 }
 
+fn try_item_collection_to_contents(items: &ItemCollection) -> Option<Vec<Contents>> {
+    items.iter().map(|(item, _)| item.try_into().ok()).collect()
+}
+
+fn try_item_collection_to_entities(items: &ItemCollection) -> Option<Vec<EntityPath>> {
+    items
+        .iter()
+        .map(|(item, _)| match item {
+            Item::InstancePath(instance_path) | Item::DataResult(_, instance_path) => instance_path
+                .is_all()
+                .then(|| instance_path.entity_path.clone()),
+            _ => None,
+        })
+        .collect()
+}
+
 impl std::fmt::Display for DragAndDropPayload {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let mut item_counter = ItemCounter::default();
+
         match self {
-            Self::Contents { contents } => items_to_string(
-                contents
-                    .iter()
-                    .map(|content| content.as_item())
-                    .collect_vec()
-                    .iter(),
-            )
-            .fmt(f),
+            Self::Contents { contents } => {
+                for content in contents {
+                    item_counter.add(&content.as_item());
+                }
+            }
+
+            Self::Entities { entities } => {
+                for entity in entities {
+                    item_counter.add(&Item::InstancePath(InstancePath::from(entity.clone())));
+                }
+            }
 
             // this is not used in the UI
-            Self::Invalid => "invalid selection".fmt(f),
+            Self::Invalid => {}
         }
+
+        item_counter.fmt(f)
     }
 }
 
@@ -59,6 +88,7 @@ pub fn drag_and_drop_payload_cursor_ui(ctx: &egui::Context) {
         if let Some(pointer_pos) = ctx.pointer_interact_pos() {
             let icon = match payload.as_ref() {
                 DragAndDropPayload::Contents { .. } => &re_ui::icons::DND_MOVE,
+                DragAndDropPayload::Entities { .. } => &re_ui::icons::DND_ADD_TO_EXISTING,
                 // don't draw anything for invalid selection
                 DragAndDropPayload::Invalid => return,
             };
@@ -76,21 +106,18 @@ pub fn drag_and_drop_payload_cursor_ui(ctx: &egui::Context) {
 
             ui.set_opacity(0.7);
 
-            let response = drag_pill_frame(matches!(
-                payload.as_ref(),
-                &DragAndDropPayload::Invalid { .. }
-            ))
-            .show(&mut ui, |ui| {
-                let text_color = ui.visuals().widgets.inactive.text_color();
+            let response = drag_pill_frame()
+                .show(&mut ui, |ui| {
+                    let text_color = ui.visuals().widgets.inactive.text_color();
 
-                ui.horizontal(|ui| {
-                    ui.spacing_mut().item_spacing.x = 2.0;
+                    ui.horizontal(|ui| {
+                        ui.spacing_mut().item_spacing.x = 2.0;
 
-                    ui.small_icon(icon, Some(text_color));
-                    ui.label(egui::RichText::new(payload.to_string()).color(text_color));
-                });
-            })
-            .response;
+                        ui.small_icon(icon, Some(text_color));
+                        ui.label(egui::RichText::new(payload.to_string()).color(text_color));
+                    });
+                })
+                .response;
 
             let delta = pointer_pos - response.rect.right_bottom();
             ctx.transform_layer_shapes(layer_id, emath::TSTransform::from_translation(delta));
@@ -98,8 +125,8 @@ pub fn drag_and_drop_payload_cursor_ui(ctx: &egui::Context) {
     }
 }
 
-fn drag_pill_frame(error_state: bool) -> egui::Frame {
-    let hue = if error_state { Hue::Red } else { Hue::Blue };
+fn drag_pill_frame() -> egui::Frame {
+    let hue = Hue::Blue;
 
     egui::Frame {
         fill: re_ui::design_tokens().color(ColorToken::new(hue, S325)),
@@ -117,62 +144,70 @@ fn drag_pill_frame(error_state: bool) -> egui::Frame {
         ..Default::default()
     }
 }
+/// Helper class to count item types and display them in a human-readable way.
+#[derive(Debug, Default)]
+struct ItemCounter {
+    container_cnt: u32,
+    view_cnt: u32,
+    app_cnt: u32,
+    data_source_cnt: u32,
+    store_cnt: u32,
+    entity_cnt: u32,
+    instance_cnt: u32,
+    component_cnt: u32,
+}
 
-fn items_to_string<'a>(items: impl Iterator<Item = &'a Item>) -> String {
-    let mut container_cnt = 0u32;
-    let mut view_cnt = 0u32;
-    let mut app_cnt = 0u32;
-    let mut data_source_cnt = 0u32;
-    let mut store_cnt = 0u32;
-    let mut entity_cnt = 0u32;
-    let mut instance_cnt = 0u32;
-    let mut component_cnt = 0u32;
-
-    for item in items {
+impl ItemCounter {
+    fn add(&mut self, item: &Item) {
         match item {
-            Item::Container(_) => container_cnt += 1,
-            Item::View(_) => view_cnt += 1,
-            Item::AppId(_) => app_cnt += 1,
-            Item::DataSource(_) => data_source_cnt += 1,
-            Item::StoreId(_) => store_cnt += 1,
+            Item::Container(_) => self.container_cnt += 1,
+            Item::View(_) => self.view_cnt += 1,
+            Item::AppId(_) => self.app_cnt += 1,
+            Item::DataSource(_) => self.data_source_cnt += 1,
+            Item::StoreId(_) => self.store_cnt += 1,
             Item::InstancePath(instance_path) | Item::DataResult(_, instance_path) => {
                 if instance_path.is_all() {
-                    entity_cnt += 1;
+                    self.entity_cnt += 1;
                 } else {
-                    instance_cnt += 1;
+                    self.instance_cnt += 1;
                 }
             }
-            Item::ComponentPath(_) => component_cnt += 1,
+            Item::ComponentPath(_) => self.component_cnt += 1,
         }
     }
+}
 
-    let count_and_names = [
-        (container_cnt, "container", "containers"),
-        (view_cnt, "view", "views"),
-        (app_cnt, "app", "apps"),
-        (data_source_cnt, "data source", "data sources"),
-        (store_cnt, "store", "stores"),
-        (entity_cnt, "entity", "entities"),
-        (instance_cnt, "instance", "instances"),
-        (component_cnt, "component", "components"),
-    ];
-
-    count_and_names
-        .into_iter()
-        .filter_map(|(count, name_singular, name_plural)| {
-            if count > 0 {
-                Some(format!(
-                    "{} {}",
-                    re_format::format_uint(count),
-                    if count == 1 {
-                        name_singular
-                    } else {
-                        name_plural
-                    },
-                ))
-            } else {
-                None
-            }
-        })
-        .join(", ")
+impl Display for ItemCounter {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let count_and_names = [
+            (&self.container_cnt, "container", "containers"),
+            (&self.view_cnt, "view", "views"),
+            (&self.app_cnt, "app", "apps"),
+            (&self.data_source_cnt, "data source", "data sources"),
+            (&self.store_cnt, "store", "stores"),
+            (&self.entity_cnt, "entity", "entities"),
+            (&self.instance_cnt, "instance", "instances"),
+            (&self.component_cnt, "component", "components"),
+        ];
+
+        count_and_names
+            .into_iter()
+            .filter_map(|(&count, name_singular, name_plural)| {
+                if count > 0 {
+                    Some(format!(
+                        "{} {}",
+                        re_format::format_uint(count),
+                        if count == 1 {
+                            name_singular
+                        } else {
+                            name_plural
+                        },
+                    ))
+                } else {
+                    None
+                }
+            })
+            .join(", ")
+            .fmt(f)
+    }
 }
diff --git a/crates/viewer/re_viewport/src/viewport_ui.rs b/crates/viewer/re_viewport/src/viewport_ui.rs
index 96382c59bd12..8b0d43749326 100644
--- a/crates/viewer/re_viewport/src/viewport_ui.rs
+++ b/crates/viewer/re_viewport/src/viewport_ui.rs
@@ -6,10 +6,12 @@ use ahash::HashMap;
 use egui_tiles::{Behavior as _, EditAction};
 
 use re_context_menu::{context_menu_ui_for_item, SelectionUpdateBehavior};
-use re_ui::{ContextExt as _, DesignTokens, Icon, UiExt as _};
+use re_log_types::{EntityPath, EntityPathRule};
+use re_ui::{design_tokens, ContextExt as _, DesignTokens, Icon, UiExt as _};
 use re_viewer_context::{
-    blueprint_id_to_tile_id, icon_for_container_kind, Contents, Item, PublishedViewInfo,
-    SystemExecutionOutput, ViewClassRegistry, ViewId, ViewQuery, ViewStates, ViewerContext,
+    blueprint_id_to_tile_id, icon_for_container_kind, Contents, DragAndDropPayload, Item,
+    PublishedViewInfo, SystemExecutionOutput, ViewClassRegistry, ViewId, ViewQuery, ViewStates,
+    ViewerContext,
 };
 use re_viewport_blueprint::{ViewportBlueprint, ViewportCommand};
 
@@ -100,6 +102,15 @@ impl ViewportUi {
 
             tree.ui(&mut egui_tiles_delegate, ui);
 
+            let dragged_payload = egui::DragAndDrop::payload::<DragAndDropPayload>(ui.ctx());
+            let dragged_payload = dragged_payload.as_ref().and_then(|payload| {
+                if let DragAndDropPayload::Entities { entities } = payload.as_ref() {
+                    Some(entities)
+                } else {
+                    None
+                }
+            });
+
             // Outline hovered & selected tiles:
             for contents in blueprint.contents_iter() {
                 let tile_id = contents.as_tile_id();
@@ -117,7 +128,16 @@ impl ViewportUi {
                         hovered = false;
                     }
 
-                    let stroke = if hovered {
+                    let view_id = contents.as_view_id();
+
+                    //TODO: add visualizabilty criteron here
+                    let candidate_drop_of_entities_in_view = dragged_payload
+                        .and_then(|payload| ui.rect_contains_pointer(rect).then_some(payload))
+                        .and_then(|payload| view_id.map(|view_id| (view_id, payload)));
+
+                    let stroke = if candidate_drop_of_entities_in_view.is_some() {
+                        design_tokens().drop_target_container_stroke()
+                    } else if hovered {
                         ui.ctx().hover_stroke()
                     } else if selected {
                         ui.ctx().selection_stroke()
@@ -125,14 +145,21 @@ impl ViewportUi {
                         continue;
                     };
 
+                    if let Some((view_id, entities)) = candidate_drop_of_entities_in_view {
+                        self.handle_entities_drop(ctx, ui, view_id, entities);
+                    }
+
                     // We want the rectangle to be on top of everything in the viewport,
                     // including stuff in "zoom-pan areas", like we use in the graph view.
-                    let top_layer_id = egui::LayerId::new(ui.layer_id().order, ui.id().with("child_id"));
+                    let top_layer_id =
+                        egui::LayerId::new(ui.layer_id().order, ui.id().with("child_id"));
                     ui.ctx().set_sublayer(ui.layer_id(), top_layer_id); // Make sure it is directly on top of the ui layer
 
                     // We need to shrink a bit so the panel-resize lines don't cover the highlight rectangle.
                     // This is hacky.
-                    ui.painter().clone().with_layer_id(top_layer_id)
+                    ui.painter()
+                        .clone()
+                        .with_layer_id(top_layer_id)
                         .rect_stroke(rect.shrink(stroke.width), 0.0, stroke);
                 }
             }
@@ -144,7 +171,8 @@ impl ViewportUi {
                 if egui_tiles_delegate.edited || is_dragging_a_tile {
                     if blueprint.auto_layout() {
                         re_log::trace!(
-                            "The user is manipulating the egui_tiles tree - will no longer auto-layout"
+                            "The user is manipulating the egui_tiles tree - will no longer \
+                            auto-layout"
                         );
                     }
 
@@ -164,7 +192,10 @@ impl ViewportUi {
                         });
                     }
 
-                    self.blueprint.deferred_commands.lock().push(ViewportCommand::SetTree(tree));
+                    self.blueprint
+                        .deferred_commands
+                        .lock()
+                        .push(ViewportCommand::SetTree(tree));
                 }
             }
         });
@@ -172,6 +203,30 @@ impl ViewportUi {
         self.blueprint.set_maximized(maximized, ctx);
     }
 
+    fn handle_entities_drop(
+        &self,
+        ctx: &ViewerContext<'_>,
+        ui: &mut egui::Ui,
+        view_id: ViewId,
+        entities: &[EntityPath],
+    ) {
+        if !ui.input(|i| i.pointer.any_released()) {
+            return;
+        }
+
+        egui::DragAndDrop::clear_payload(ui.ctx());
+
+        let Some(view) = self.blueprint.view(&view_id) else {
+            return;
+        };
+
+        for entity in entities {
+            //TODO: do that only visualizable
+            view.contents
+                .raw_add_entity_inclusion(ctx, EntityPathRule::including_subtree(entity.clone()));
+        }
+    }
+
     pub fn on_frame_start(&self, ctx: &ViewerContext<'_>) {
         re_tracing::profile_function!();
 

From 9ed096f0a68f41485c42c5ad81fcb9f7d1ae02a6 Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Wed, 11 Dec 2024 17:28:24 +0100
Subject: [PATCH 02/16] Fix maximized view not getting highlighted

---
 crates/viewer/re_viewport/src/viewport_ui.rs | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/crates/viewer/re_viewport/src/viewport_ui.rs b/crates/viewer/re_viewport/src/viewport_ui.rs
index 8b0d43749326..d3454b22d78a 100644
--- a/crates/viewer/re_viewport/src/viewport_ui.rs
+++ b/crates/viewer/re_viewport/src/viewport_ui.rs
@@ -70,8 +70,12 @@ impl ViewportUi {
 
         let mut tree = if let Some(view_id) = blueprint.maximized {
             let mut tiles = egui_tiles::Tiles::default();
-            let root = tiles.insert_pane(view_id);
-            egui_tiles::Tree::new("viewport_tree", root, tiles)
+
+            // we must ensure that our temporary tree has the correct tile id, such that the tile id
+            // to view id logic later in this function works correctly
+            let tile_id = Contents::View(view_id).as_tile_id();
+            tiles.insert(tile_id, egui_tiles::Tile::Pane(view_id));
+            egui_tiles::Tree::new("viewport_tree", tile_id, tiles)
         } else {
             blueprint.tree.clone()
         };

From 56bb57f75ffe7fde1c37e705b1cd3ce6958f4374 Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Wed, 11 Dec 2024 17:50:31 +0100
Subject: [PATCH 03/16] Dont unnecessarily pass a `ui`

---
 crates/viewer/re_viewport/src/viewport_ui.rs | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/crates/viewer/re_viewport/src/viewport_ui.rs b/crates/viewer/re_viewport/src/viewport_ui.rs
index d3454b22d78a..83b9d6ff38b2 100644
--- a/crates/viewer/re_viewport/src/viewport_ui.rs
+++ b/crates/viewer/re_viewport/src/viewport_ui.rs
@@ -150,7 +150,7 @@ impl ViewportUi {
                     };
 
                     if let Some((view_id, entities)) = candidate_drop_of_entities_in_view {
-                        self.handle_entities_drop(ctx, ui, view_id, entities);
+                        self.handle_entities_drop(ctx, view_id, entities);
                     }
 
                     // We want the rectangle to be on top of everything in the viewport,
@@ -210,15 +210,14 @@ impl ViewportUi {
     fn handle_entities_drop(
         &self,
         ctx: &ViewerContext<'_>,
-        ui: &mut egui::Ui,
         view_id: ViewId,
         entities: &[EntityPath],
     ) {
-        if !ui.input(|i| i.pointer.any_released()) {
+        if !ctx.egui_ctx.input(|i| i.pointer.any_released()) {
             return;
         }
 
-        egui::DragAndDrop::clear_payload(ui.ctx());
+        egui::DragAndDrop::clear_payload(ctx.egui_ctx);
 
         let Some(view) = self.blueprint.view(&view_id) else {
             return;

From 6ddf6cfd72c2a8044a754b766485796b15313754 Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Wed, 11 Dec 2024 17:53:47 +0100
Subject: [PATCH 04/16] Move the EntityAddInfo to re_viewport_blueprint to make
 it availalble to other crates

---
 .../src/view_entity_picker.rs                 | 119 +----------------
 .../src/entity_add_info.rs                    | 121 ++++++++++++++++++
 .../viewer/re_viewport_blueprint/src/lib.rs   |   2 +
 3 files changed, 127 insertions(+), 115 deletions(-)
 create mode 100644 crates/viewer/re_viewport_blueprint/src/entity_add_info.rs

diff --git a/crates/viewer/re_selection_panel/src/view_entity_picker.rs b/crates/viewer/re_selection_panel/src/view_entity_picker.rs
index ba1f814076de..7ee7cacae4f5 100644
--- a/crates/viewer/re_selection_panel/src/view_entity_picker.rs
+++ b/crates/viewer/re_selection_panel/src/view_entity_picker.rs
@@ -5,8 +5,10 @@ use re_data_ui::item_ui;
 use re_entity_db::{EntityPath, EntityTree, InstancePath};
 use re_log_types::{EntityPathFilter, EntityPathRule};
 use re_ui::UiExt as _;
-use re_viewer_context::{DataQueryResult, ViewClassExt as _, ViewId, ViewerContext};
-use re_viewport_blueprint::{ViewBlueprint, ViewportBlueprint};
+use re_viewer_context::{DataQueryResult, ViewId, ViewerContext};
+use re_viewport_blueprint::{
+    create_entity_add_info, CanAddToView, EntityAddInfo, ViewBlueprint, ViewportBlueprint,
+};
 
 /// Window for adding/removing entities from a view.
 ///
@@ -245,116 +247,3 @@ fn add_entities_line_ui(
         });
     });
 }
-
-/// Describes if an entity path can be added to a view.
-#[derive(Clone, PartialEq, Eq)]
-enum CanAddToView {
-    Compatible { already_added: bool },
-    No { reason: String },
-}
-
-impl Default for CanAddToView {
-    fn default() -> Self {
-        Self::Compatible {
-            already_added: false,
-        }
-    }
-}
-
-impl CanAddToView {
-    /// Can be generally added but view might already have this element.
-    pub fn is_compatible(&self) -> bool {
-        match self {
-            Self::Compatible { .. } => true,
-            Self::No { .. } => false,
-        }
-    }
-
-    /// Can be added and view doesn't have it already.
-    pub fn is_compatible_and_missing(&self) -> bool {
-        self == &Self::Compatible {
-            already_added: false,
-        }
-    }
-
-    pub fn join(&self, other: &Self) -> Self {
-        match self {
-            Self::Compatible { already_added } => {
-                let already_added = if let Self::Compatible {
-                    already_added: already_added_other,
-                } = other
-                {
-                    *already_added && *already_added_other
-                } else {
-                    *already_added
-                };
-                Self::Compatible { already_added }
-            }
-            Self::No { .. } => other.clone(),
-        }
-    }
-}
-
-#[derive(Default)]
-#[allow(dead_code)]
-struct EntityAddInfo {
-    can_add: CanAddToView,
-    can_add_self_or_descendant: CanAddToView,
-}
-
-fn create_entity_add_info(
-    ctx: &ViewerContext<'_>,
-    tree: &EntityTree,
-    view: &ViewBlueprint,
-    query_result: &DataQueryResult,
-) -> IntMap<EntityPath, EntityAddInfo> {
-    let mut meta_data: IntMap<EntityPath, EntityAddInfo> = IntMap::default();
-
-    // TODO(andreas): This should be state that is already available because it's part of the view's state.
-    let class = view.class(ctx.view_class_registry);
-    let visualizable_entities = class.determine_visualizable_entities(
-        ctx.applicable_entities_per_visualizer,
-        ctx.recording(),
-        &ctx.view_class_registry
-            .new_visualizer_collection(view.class_identifier()),
-        &view.space_origin,
-    );
-
-    tree.visit_children_recursively(|entity_path| {
-        let can_add: CanAddToView =
-            if visualizable_entities.iter().any(|(_, entities)| entities.contains(entity_path)) {
-                CanAddToView::Compatible {
-                    already_added: query_result.contains_entity(entity_path),
-                }
-            } else {
-                // TODO(#6321): This shouldn't necessarily prevent us from adding it.
-                CanAddToView::No {
-                    reason: format!(
-                        "Entity can't be displayed by any of the available visualizers in this class of view ({}).",
-                        view.class_identifier()
-                    ),
-                }
-            };
-
-        if can_add.is_compatible() {
-            // Mark parents aware that there is some descendant that is compatible
-            let mut path = entity_path.clone();
-            while let Some(parent) = path.parent() {
-                let data = meta_data.entry(parent.clone()).or_default();
-                data.can_add_self_or_descendant = data.can_add_self_or_descendant.join(&can_add);
-                path = parent;
-            }
-        }
-
-        let can_add_self_or_descendant = can_add.clone();
-        meta_data.insert(
-            entity_path.clone(),
-            EntityAddInfo {
-                can_add,
-                can_add_self_or_descendant,
-            },
-        );
-    });
-
-    meta_data
-}
diff --git a/crates/viewer/re_viewport_blueprint/src/entity_add_info.rs b/crates/viewer/re_viewport_blueprint/src/entity_add_info.rs
new file mode 100644
index 000000000000..7d4aa4825d8f
--- /dev/null
+++ b/crates/viewer/re_viewport_blueprint/src/entity_add_info.rs
@@ -0,0 +1,121 @@
+//! Utilities for determining if an entity can be added to a view.
+
+use nohash_hasher::IntMap;
+
+use re_entity_db::EntityTree;
+use re_log_types::EntityPath;
+use re_viewer_context::{DataQueryResult, ViewClassExt as _, ViewerContext};
+
+use crate::ViewBlueprint;
+
+/// Describes if an entity path can be added to a view.
+#[derive(Clone, PartialEq, Eq)]
+pub enum CanAddToView {
+    Compatible { already_added: bool },
+    No { reason: String },
+}
+
+impl Default for CanAddToView {
+    fn default() -> Self {
+        Self::Compatible {
+            already_added: false,
+        }
+    }
+}
+
+impl CanAddToView {
+    /// Can be generally added but view might already have this element.
+    pub fn is_compatible(&self) -> bool {
+        match self {
+            Self::Compatible { .. } => true,
+            Self::No { .. } => false,
+        }
+    }
+
+    /// Can be added and view doesn't have it already.
+    pub fn is_compatible_and_missing(&self) -> bool {
+        self == &Self::Compatible {
+            already_added: false,
+        }
+    }
+
+    pub fn join(&self, other: &Self) -> Self {
+        match self {
+            Self::Compatible { already_added } => {
+                let already_added = if let Self::Compatible {
+                    already_added: already_added_other,
+                } = other
+                {
+                    *already_added && *already_added_other
+                } else {
+                    *already_added
+                };
+                Self::Compatible { already_added }
+            }
+            Self::No { .. } => other.clone(),
+        }
+    }
+}
+
+#[derive(Default)]
+pub struct EntityAddInfo {
+    pub can_add: CanAddToView,
+    pub can_add_self_or_descendant: CanAddToView,
+}
+
+pub fn create_entity_add_info(
+    ctx: &ViewerContext<'_>,
+    tree: &EntityTree,
+    view: &ViewBlueprint,
+    query_result: &DataQueryResult,
+) -> IntMap<EntityPath, EntityAddInfo> {
+    let mut meta_data: IntMap<EntityPath, EntityAddInfo> = IntMap::default();
+
+    // TODO(andreas): This should be state that is already available because it's part of the view's state.
+    let class = view.class(ctx.view_class_registry);
+    let visualizable_entities = class.determine_visualizable_entities(
+        ctx.applicable_entities_per_visualizer,
+        ctx.recording(),
+        &ctx.view_class_registry
+            .new_visualizer_collection(view.class_identifier()),
+        &view.space_origin,
+    );
+
+    tree.visit_children_recursively(|entity_path| {
+        let can_add: CanAddToView =
+            if visualizable_entities.iter().any(|(_, entities)| entities.contains(entity_path)) {
+                CanAddToView::Compatible {
+                    already_added: query_result.contains_entity(entity_path),
+                }
+            } else {
+                // TODO(#6321): This shouldn't necessarily prevent us from adding it.
+                CanAddToView::No {
+                    reason: format!(
+                        "Entity can't be displayed by any of the available visualizers in this class of view ({}).",
+                        view.class_identifier()
+                    ),
+                }
+            };
+
+        if can_add.is_compatible() {
+            // Mark parents aware that there is some descendant that is compatible
+            let mut path = entity_path.clone();
+            while let Some(parent) = path.parent() {
+                let data = meta_data.entry(parent.clone()).or_default();
+                data.can_add_self_or_descendant = data.can_add_self_or_descendant.join(&can_add);
+                path = parent;
+            }
+        }
+
+        let can_add_self_or_descendant = can_add.clone();
+        meta_data.insert(
+            entity_path.clone(),
+            EntityAddInfo {
+                can_add,
+                can_add_self_or_descendant,
+            },
+        );
+    });
+
+    meta_data
+}
diff --git a/crates/viewer/re_viewport_blueprint/src/lib.rs b/crates/viewer/re_viewport_blueprint/src/lib.rs
index 3ec02beb7356..a10abbcce1ea 100644
--- a/crates/viewer/re_viewport_blueprint/src/lib.rs
+++ b/crates/viewer/re_viewport_blueprint/src/lib.rs
@@ -3,6 +3,7 @@
 //! This crate provides blueprint (i.e. description) for how to render the viewport.
 
 mod container;
+mod entity_add_info;
 pub mod ui;
 mod view;
 mod view_contents;
@@ -11,6 +12,7 @@ mod viewport_blueprint;
 mod viewport_command;
 
 pub use container::ContainerBlueprint;
+pub use entity_add_info::{create_entity_add_info, CanAddToView, EntityAddInfo};
 use re_viewer_context::ViewerContext;
 pub use view::ViewBlueprint;
 pub use view_contents::ViewContents;

From 62f5f5b9d9fb2038cb612a964d412bb495047f3f Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Thu, 12 Dec 2024 10:09:19 +0100
Subject: [PATCH 05/16] well, just don't... :shrug:

---
 crates/viewer/re_selection_panel/src/view_entity_picker.rs | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/crates/viewer/re_selection_panel/src/view_entity_picker.rs b/crates/viewer/re_selection_panel/src/view_entity_picker.rs
index 7ee7cacae4f5..52e66f1dfcc8 100644
--- a/crates/viewer/re_selection_panel/src/view_entity_picker.rs
+++ b/crates/viewer/re_selection_panel/src/view_entity_picker.rs
@@ -58,10 +58,9 @@ fn add_entities_ui(ctx: &ViewerContext<'_>, ui: &mut egui::Ui, view: &ViewBluepr
     re_tracing::profile_function!();
 
     let tree = &ctx.recording().tree();
-    // TODO(jleibs): Avoid clone
-    let query_result = ctx.lookup_query_result(view.id).clone();
+    let query_result = ctx.lookup_query_result(view.id);
     let entity_path_filter = &view.contents.entity_path_filter;
-    let entities_add_info = create_entity_add_info(ctx, tree, view, &query_result);
+    let entities_add_info = create_entity_add_info(ctx, tree, view, query_result);
 
     add_entities_tree_ui(
         ctx,
@@ -69,7 +68,7 @@ fn add_entities_ui(ctx: &ViewerContext<'_>, ui: &mut egui::Ui, view: &ViewBluepr
         &tree.path.to_string(),
         tree,
         view,
-        &query_result,
+        query_result,
         entity_path_filter,
         &entities_add_info,
     );

From 16c3fe87f43445baf06d4d45029068fa918ce8e4 Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Thu, 12 Dec 2024 11:13:51 +0100
Subject: [PATCH 06/16] consider visualizability when highlighting target view
 and adding entities to filter

---
 crates/viewer/re_viewport/src/viewport_ui.rs | 92 ++++++++++++++------
 1 file changed, 67 insertions(+), 25 deletions(-)

diff --git a/crates/viewer/re_viewport/src/viewport_ui.rs b/crates/viewer/re_viewport/src/viewport_ui.rs
index 83b9d6ff38b2..7a051793a01f 100644
--- a/crates/viewer/re_viewport/src/viewport_ui.rs
+++ b/crates/viewer/re_viewport/src/viewport_ui.rs
@@ -13,7 +13,9 @@ use re_viewer_context::{
     PublishedViewInfo, SystemExecutionOutput, ViewClassRegistry, ViewId, ViewQuery, ViewStates,
     ViewerContext,
 };
-use re_viewport_blueprint::{ViewportBlueprint, ViewportCommand};
+use re_viewport_blueprint::{
+    create_entity_add_info, ViewBlueprint, ViewportBlueprint, ViewportCommand,
+};
 
 use crate::system_execution::{execute_systems_for_all_views, execute_systems_for_view};
 
@@ -132,14 +134,28 @@ impl ViewportUi {
                         hovered = false;
                     }
 
-                    let view_id = contents.as_view_id();
+                    // Handle drag-and-drop if this is a view.
+                    //TODO(#8428): simplify with let-chains
+                    let should_display_drop_destination_frame = 'scope: {
+                        if !ui.rect_contains_pointer(rect) {
+                            break 'scope false;
+                        }
+
+                        let Some(view_blueprint) = contents
+                            .as_view_id()
+                            .and_then(|view_id| self.blueprint.view(&view_id))
+                        else {
+                            break 'scope false;
+                        };
 
-                    //TODO: add visualizabilty criteron here
-                    let candidate_drop_of_entities_in_view = dragged_payload
-                        .and_then(|payload| ui.rect_contains_pointer(rect).then_some(payload))
-                        .and_then(|payload| view_id.map(|view_id| (view_id, payload)));
+                        let Some(dragged_payload) = dragged_payload else {
+                            break 'scope false;
+                        };
 
-                    let stroke = if candidate_drop_of_entities_in_view.is_some() {
+                        Self::handle_drop_entities_to_view(ctx, view_blueprint, dragged_payload)
+                    };
+
+                    let stroke = if should_display_drop_destination_frame {
                         design_tokens().drop_target_container_stroke()
                     } else if hovered {
                         ui.ctx().hover_stroke()
@@ -149,10 +165,6 @@ impl ViewportUi {
                         continue;
                     };
 
-                    if let Some((view_id, entities)) = candidate_drop_of_entities_in_view {
-                        self.handle_entities_drop(ctx, view_id, entities);
-                    }
-
                     // We want the rectangle to be on top of everything in the viewport,
                     // including stuff in "zoom-pan areas", like we use in the graph view.
                     let top_layer_id =
@@ -207,26 +219,56 @@ impl ViewportUi {
         self.blueprint.set_maximized(maximized, ctx);
     }
 
-    fn handle_entities_drop(
-        &self,
+    /// Handle the entities being dragged over a view.
+    ///
+    /// Returns whether a "drop zone candidate" frame should be displayed to the user.
+    ///
+    /// Design decisions:
+    /// - We accept the drop only if at least one of the entities is visualizable and not already
+    ///   included.
+    /// - When the drop happens, of all dropped entities, we only add those which are visualizable.
+    ///
+    fn handle_drop_entities_to_view(
         ctx: &ViewerContext<'_>,
-        view_id: ViewId,
+        view_blueprint: &ViewBlueprint,
         entities: &[EntityPath],
-    ) {
-        if !ctx.egui_ctx.input(|i| i.pointer.any_released()) {
-            return;
+    ) -> bool {
+        let add_info = create_entity_add_info(
+            ctx,
+            ctx.recording().tree(),
+            view_blueprint,
+            ctx.lookup_query_result(view_blueprint.id),
+        );
+
+        // check if any entity or its children are visualizable and not yet included in the view
+        let can_entity_be_added = |entity: &EntityPath| {
+            add_info
+                .get(entity)
+                .is_some_and(|info| info.can_add_self_or_descendant.is_compatible_and_missing())
+        };
+
+        let any_is_visualizable = entities.iter().any(can_entity_be_added);
+        if !any_is_visualizable {
+            return false;
         }
 
-        egui::DragAndDrop::clear_payload(ctx.egui_ctx);
+        // drop incoming!
+        if ctx.egui_ctx.input(|i| i.pointer.any_released()) {
+            egui::DragAndDrop::clear_payload(ctx.egui_ctx);
 
-        let Some(view) = self.blueprint.view(&view_id) else {
-            return;
-        };
+            for entity in entities {
+                if can_entity_be_added(entity) {
+                    view_blueprint.contents.raw_add_entity_inclusion(
+                        ctx,
+                        EntityPathRule::including_subtree(entity.clone()),
+                    );
+                }
+            }
 
-        for entity in entities {
-            //TODO: do that only visualizable
-            view.contents
-                .raw_add_entity_inclusion(ctx, EntityPathRule::including_subtree(entity.clone()));
+            // drop is completed, no need for highlighting anymore
+            false
+        } else {
+            any_is_visualizable
         }
     }
 

From 9df6e8b9bf03f97683008ef09598de32ee6ca1f4 Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Thu, 12 Dec 2024 11:30:44 +0100
Subject: [PATCH 07/16] lint

---
 crates/viewer/re_viewer_context/src/drag_and_drop.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/crates/viewer/re_viewer_context/src/drag_and_drop.rs b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
index 63bc95fff5bc..8864e53760b5 100644
--- a/crates/viewer/re_viewer_context/src/drag_and_drop.rs
+++ b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
@@ -144,6 +144,7 @@ fn drag_pill_frame() -> egui::Frame {
         ..Default::default()
     }
 }
+
 /// Helper class to count item types and display them in a human-readable way.
 #[derive(Debug, Default)]
 struct ItemCounter {

From b7f267babd505da3c1c93dae2f68118e208804ee Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Thu, 12 Dec 2024 15:52:03 +0100
Subject: [PATCH 08/16] select view on successful drop + slightly dim view with
 blue color when they can accept a drop

---
 crates/viewer/re_viewport/src/viewport_ui.rs | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/crates/viewer/re_viewport/src/viewport_ui.rs b/crates/viewer/re_viewport/src/viewport_ui.rs
index 7a051793a01f..804f3e14a24e 100644
--- a/crates/viewer/re_viewport/src/viewport_ui.rs
+++ b/crates/viewer/re_viewport/src/viewport_ui.rs
@@ -173,10 +173,16 @@ impl ViewportUi {
 
                     // We need to shrink a bit so the panel-resize lines don't cover the highlight rectangle.
                     // This is hacky.
-                    ui.painter()
-                        .clone()
-                        .with_layer_id(top_layer_id)
-                        .rect_stroke(rect.shrink(stroke.width), 0.0, stroke);
+                    let painter = ui.painter().clone().with_layer_id(top_layer_id);
+                    painter.rect_stroke(rect.shrink(stroke.width), 0.0, stroke);
+
+                    if should_display_drop_destination_frame {
+                        painter.rect_filled(
+                            rect.shrink(stroke.width),
+                            0.0,
+                            stroke.color.gamma_multiply(0.1),
+                        );
+                    }
                 }
             }
 
@@ -265,6 +271,9 @@ impl ViewportUi {
                 }
             }
 
+            ctx.selection_state()
+                .set_selection(Item::View(view_blueprint.id));
+
             // drop is completed, no need for highlighting anymore
             false
         } else {

From 4539d4e6cac29d92611bb7f81c1947ccd2ecde7c Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Thu, 12 Dec 2024 16:04:13 +0100
Subject: [PATCH 09/16] make drop target frame 2px wide (+ fix display bug in
 the blueprint tree)

---
 crates/viewer/re_ui/src/design_tokens.rs       | 2 +-
 crates/viewer/re_ui/src/list_item/list_item.rs | 7 ++-----
 crates/viewer/re_ui/src/ui_ext.rs              | 6 +++++-
 3 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/crates/viewer/re_ui/src/design_tokens.rs b/crates/viewer/re_ui/src/design_tokens.rs
index 8d6a6f8a53f3..125e64989d93 100644
--- a/crates/viewer/re_ui/src/design_tokens.rs
+++ b/crates/viewer/re_ui/src/design_tokens.rs
@@ -439,7 +439,7 @@ impl DesignTokens {
     /// being hovered (e.g., a container in the blueprint tree)
     #[inline]
     pub fn drop_target_container_stroke(&self) -> egui::Stroke {
-        egui::Stroke::new(1.0, self.color(ColorToken::blue(S350)))
+        egui::Stroke::new(2.0, self.color(ColorToken::blue(S350)))
     }
 }
 
diff --git a/crates/viewer/re_ui/src/list_item/list_item.rs b/crates/viewer/re_ui/src/list_item/list_item.rs
index 275505a00659..721f740851ac 100644
--- a/crates/viewer/re_ui/src/list_item/list_item.rs
+++ b/crates/viewer/re_ui/src/list_item/list_item.rs
@@ -384,13 +384,10 @@ impl ListItem {
             let bg_rect_to_paint = ui.painter().round_rect_to_pixels(bg_rect);
 
             if drag_target {
+                let stroke = crate::design_tokens().drop_target_container_stroke();
                 ui.painter().set(
                     background_frame,
-                    Shape::rect_stroke(
-                        bg_rect_to_paint.expand(-1.0),
-                        0.0,
-                        crate::design_tokens().drop_target_container_stroke(),
-                    ),
+                    Shape::rect_stroke(bg_rect_to_paint.shrink(stroke.width), 0.0, stroke),
                 );
             }
 
diff --git a/crates/viewer/re_ui/src/ui_ext.rs b/crates/viewer/re_ui/src/ui_ext.rs
index 547332d0f6d3..561f772b2ab0 100644
--- a/crates/viewer/re_ui/src/ui_ext.rs
+++ b/crates/viewer/re_ui/src/ui_ext.rs
@@ -1094,7 +1094,11 @@ pub trait UiExt {
                 return *span;
             }
 
-            if node.has_visible_frame() || node.is_area_ui() || node.is_root_ui() {
+            if node.has_visible_frame()
+                || node.is_area_ui()
+                || node.is_panel_ui()
+                || node.is_root_ui()
+            {
                 return (node.max_rect + node.frame().inner_margin).x_range();
             }
         }

From e6d4713bfcd6ad11fe5fdf188e3914ab076a74de Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Thu, 12 Dec 2024 19:06:28 +0100
Subject: [PATCH 10/16] have the pill feedback be controllable from the drop
 location code. pill now looks gray when not droppable.

---
 .../re_blueprint_tree/src/blueprint_tree.rs   |  24 ++-
 crates/viewer/re_viewer/src/app_state.rs      |  15 +-
 .../re_viewer_context/src/drag_and_drop.rs    | 198 ++++++++++++++----
 crates/viewer/re_viewer_context/src/lib.rs    |   2 +-
 .../re_viewer_context/src/test_context.rs     |   8 +-
 .../re_viewer_context/src/viewer_context.rs   |  18 +-
 crates/viewer/re_viewport/src/viewport_ui.rs  |  14 +-
 7 files changed, 202 insertions(+), 77 deletions(-)

diff --git a/crates/viewer/re_blueprint_tree/src/blueprint_tree.rs b/crates/viewer/re_blueprint_tree/src/blueprint_tree.rs
index d22b70d8c987..3af575f1da63 100644
--- a/crates/viewer/re_blueprint_tree/src/blueprint_tree.rs
+++ b/crates/viewer/re_blueprint_tree/src/blueprint_tree.rs
@@ -10,7 +10,7 @@ use re_types::blueprint::components::Visible;
 use re_ui::{drag_and_drop::DropTarget, list_item, ContextExt as _, DesignTokens, UiExt as _};
 use re_viewer_context::{
     contents_name_style, icon_for_container_kind, CollapseScope, Contents, DataResultNodeOrPath,
-    DragAndDropPayload, SystemCommandSender,
+    DragAndDropFeedback, DragAndDropPayload, SystemCommandSender,
 };
 use re_viewer_context::{
     ContainerId, DataQueryResult, DataResultNode, HoverHighlight, Item, ViewId, ViewerContext,
@@ -101,6 +101,7 @@ impl BlueprintTree {
 
                     // handle drag and drop interaction on empty space
                     self.handle_empty_space_drag_and_drop_interaction(
+                        ctx,
                         viewport,
                         ui,
                         empty_space_response.rect,
@@ -192,6 +193,7 @@ impl BlueprintTree {
         ctx.handle_select_hover_drag_interactions(&item_response, item, true);
 
         self.handle_root_container_drag_and_drop_interaction(
+            ctx,
             viewport,
             ui,
             Contents::Container(container_id),
@@ -275,6 +277,7 @@ impl BlueprintTree {
         viewport.set_content_visibility(ctx, &content, visible);
 
         self.handle_drag_and_drop_interaction(
+            ctx,
             viewport,
             ui,
             content,
@@ -411,6 +414,7 @@ impl BlueprintTree {
 
         viewport.set_content_visibility(ctx, &content, visible);
         self.handle_drag_and_drop_interaction(
+            ctx,
             viewport,
             ui,
             content,
@@ -629,6 +633,7 @@ impl BlueprintTree {
 
     fn handle_root_container_drag_and_drop_interaction(
         &mut self,
+        ctx: &ViewerContext<'_>,
         viewport: &ViewportBlueprint,
         ui: &egui::Ui,
         contents: Contents,
@@ -672,12 +677,13 @@ impl BlueprintTree {
         );
 
         if let Some(drop_target) = drop_target {
-            self.handle_contents_drop_target(viewport, ui, dragged_contents, &drop_target);
+            self.handle_contents_drop_target(ctx, viewport, ui, dragged_contents, &drop_target);
         }
     }
 
     fn handle_drag_and_drop_interaction(
         &mut self,
+        ctx: &ViewerContext<'_>,
         viewport: &ViewportBlueprint,
         ui: &egui::Ui,
         contents: Contents,
@@ -751,12 +757,13 @@ impl BlueprintTree {
         );
 
         if let Some(drop_target) = drop_target {
-            self.handle_contents_drop_target(viewport, ui, dragged_contents, &drop_target);
+            self.handle_contents_drop_target(ctx, viewport, ui, dragged_contents, &drop_target);
         }
     }
 
     fn handle_empty_space_drag_and_drop_interaction(
         &mut self,
+        ctx: &ViewerContext<'_>,
         viewport: &ViewportBlueprint,
         ui: &egui::Ui,
         empty_space: egui::Rect,
@@ -792,12 +799,13 @@ impl BlueprintTree {
                 usize::MAX,
             );
 
-            self.handle_contents_drop_target(viewport, ui, dragged_contents, &drop_target);
+            self.handle_contents_drop_target(ctx, viewport, ui, dragged_contents, &drop_target);
         }
     }
 
     fn handle_contents_drop_target(
         &mut self,
+        ctx: &ViewerContext<'_>,
         viewport: &ViewportBlueprint,
         ui: &Ui,
         dragged_contents: &[Contents],
@@ -816,6 +824,8 @@ impl BlueprintTree {
             false
         };
         if dragged_contents.iter().any(parent_contains_dragged_content) {
+            ctx.drag_and_drop_manager
+                .set_feedback(DragAndDropFeedback::Reject);
             return;
         }
 
@@ -826,7 +836,9 @@ impl BlueprintTree {
         );
 
         let Contents::Container(target_container_id) = drop_target.target_parent_id else {
-            // this shouldn't append
+            // this shouldn't happen
+            ctx.drag_and_drop_manager
+                .set_feedback(DragAndDropFeedback::Reject);
             return;
         };
 
@@ -839,6 +851,8 @@ impl BlueprintTree {
 
             egui::DragAndDrop::clear_payload(ui.ctx());
         } else {
+            ctx.drag_and_drop_manager
+                .set_feedback(DragAndDropFeedback::Accept);
             self.next_candidate_drop_parent_container_id = Some(target_container_id);
         }
     }
diff --git a/crates/viewer/re_viewer/src/app_state.rs b/crates/viewer/re_viewer/src/app_state.rs
index 1d0a1559b13f..3870f63743b0 100644
--- a/crates/viewer/re_viewer/src/app_state.rs
+++ b/crates/viewer/re_viewer/src/app_state.rs
@@ -8,8 +8,8 @@ use re_smart_channel::ReceiveSet;
 use re_types::blueprint::components::PanelState;
 use re_ui::{ContextExt as _, DesignTokens};
 use re_viewer_context::{
-    drag_and_drop_payload_cursor_ui, AppOptions, ApplicationSelectionState, BlueprintUndoState,
-    CommandSender, ComponentUiRegistry, PlayState, RecordingConfig, StoreContext, StoreHub,
+    AppOptions, ApplicationSelectionState, BlueprintUndoState, CommandSender, ComponentUiRegistry,
+    DragAndDropManager, PlayState, RecordingConfig, StoreContext, StoreHub,
     SystemCommandSender as _, ViewClassExt as _, ViewClassRegistry, ViewStates, ViewerContext,
 };
 use re_viewport::ViewportUi;
@@ -215,8 +215,9 @@ impl AppState {
         );
 
         // The root container cannot be dragged.
-        let undraggable_items =
-            re_viewer_context::Item::Container(viewport_ui.blueprint.root_container).into();
+        let drag_and_drop_manager = DragAndDropManager::new(re_viewer_context::Item::Container(
+            viewport_ui.blueprint.root_container,
+        ));
 
         let applicable_entities_per_visualizer =
             view_class_registry.applicable_entities_for_visualizer_systems(&recording.store_id());
@@ -278,7 +279,7 @@ impl AppState {
             render_ctx: Some(render_ctx),
             command_sender,
             focused_item,
-            undraggable_items: &undraggable_items,
+            drag_and_drop_manager: &drag_and_drop_manager,
         };
 
         // We move the time at the very start of the frame,
@@ -350,7 +351,7 @@ impl AppState {
             render_ctx: Some(render_ctx),
             command_sender,
             focused_item,
-            undraggable_items: &undraggable_items,
+            drag_and_drop_manager: &drag_and_drop_manager,
         };
 
         if *show_settings_ui {
@@ -517,7 +518,7 @@ impl AppState {
         //
 
         add_view_or_container_modal_ui(&ctx, &viewport_ui.blueprint, ui);
-        drag_and_drop_payload_cursor_ui(ctx.egui_ctx);
+        drag_and_drop_manager.payload_cursor_ui(ctx.egui_ctx);
 
         // Process deferred layout operations and apply updates back to blueprint:
         viewport_ui.save_to_blueprint_store(&ctx, view_class_registry);
diff --git a/crates/viewer/re_viewer_context/src/drag_and_drop.rs b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
index 8864e53760b5..8c3f2553c64f 100644
--- a/crates/viewer/re_viewer_context/src/drag_and_drop.rs
+++ b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
@@ -1,9 +1,46 @@
-//! Implement a global drag-and-drop payload type that enable dragging from various parts of the UI
-//! (e.g., from the streams tree to the viewport, etc.).
+//! Support for viewer-wide drag-and-drop of [`crate::Item`]s.
+//!
+//! ## Theory of operation
+//!
+//! ### Setup
+//!
+//! A [`DragAndDropManager`] should be created at the start of the frame and made available to the
+//! entire UI code.
+//!
+//!
+//! ### Initiating a drag
+//!
+//! Any UI representation of an [`crate::Item`] may initiate a drag.
+//! [`crate::ViewerContext::handle_select_hover_drag_interactions`] will handle that automatically
+//! when passed `true` for its `draggable` argument.
+//!
+//!
+//! ### Reacting to a drag and accepting a drop
+//!
+//! This part of the process is more involved and typically includes the following steps:
+//!
+//! 1. When hovered, the receiving UI element should check for a compatible payload using
+//!    [`egui::DragAndDrop::payload`] and matching one or more variants of the returned
+//!    [`DragAndDropPayload`], if any.
+//!
+//! 2. If an acceptable payload type is being dragged, the UI element should provide appropriate
+//!    visual feedback. This includes:
+//!    - Calling [`DragAndDropManager::set_feedback`] with the appropriate feedback.
+//!    - Drawing a frame around the target container with
+//!      [`re_ui::DesignToken::drop_target_container_stroke`].
+//!    - Optionally provide more feedback, e.g., where exactly the payload will be inserted within
+//!      the container.
+//!
+//! 3. If the mouse is released (using [`egui::PointerState::any_released`]), the payload must be
+//!    actually transferred to the container and [`egui::DragAndDrop::clear_payload`] must be
+//!    called.
 
 use std::fmt::{Display, Formatter};
+use std::sync::Arc;
 
 use itertools::Itertools;
+use parking_lot::Mutex;
+
 use re_entity_db::InstancePath;
 use re_log_types::EntityPath;
 use re_ui::{
@@ -14,7 +51,6 @@ use re_ui::{
 
 use crate::{Contents, Item, ItemCollection};
 
-//TODO(ab): add more type of things we can drag, in particular entity paths
 #[derive(Debug)]
 pub enum DragAndDropPayload {
     /// The dragged content is made only of [`Contents`].
@@ -80,53 +116,123 @@ impl std::fmt::Display for DragAndDropPayload {
     }
 }
 
-/// Display the currently dragged payload as a pill in the UI.
+#[derive(Default, Debug, Clone, Copy, PartialEq, Eq)]
+pub enum DragAndDropFeedback {
+    /// The payload type is irrelevant to me.
+    ///
+    /// For example, dropping a view and/or contain onto an existing view in the viewport is
+    /// irrelevant.
+    ///
+    /// This is the default displayed feedback, unless explicitly set otherwise by some UI hovered
+    /// UI element.
+    #[default]
+    Ignore,
+
+    /// The payload type is acceptable and could successfully be dropped at the current location.
+    Accept,
+
+    /// The payload type is correct, but it's content cannot be accepted by the current drop location.
+    ///
+    /// For example, a view might reject an entity because it already contains it.
+    Reject,
+}
+
+/// Helper to handle drag-and-drop operations.
 ///
-/// This should be called once per frame.
-pub fn drag_and_drop_payload_cursor_ui(ctx: &egui::Context) {
-    if let Some(payload) = egui::DragAndDrop::payload::<DragAndDropPayload>(ctx) {
-        if let Some(pointer_pos) = ctx.pointer_interact_pos() {
-            let icon = match payload.as_ref() {
-                DragAndDropPayload::Contents { .. } => &re_ui::icons::DND_MOVE,
-                DragAndDropPayload::Entities { .. } => &re_ui::icons::DND_ADD_TO_EXISTING,
-                // don't draw anything for invalid selection
-                DragAndDropPayload::Invalid => return,
-            };
-
-            let layer_id = egui::LayerId::new(
-                egui::Order::Tooltip,
-                egui::Id::new("drag_and_drop_payload_layer"),
-            );
-
-            let mut ui = egui::Ui::new(
-                ctx.clone(),
-                egui::Id::new("rerun_drag_and_drop_payload_ui"),
-                egui::UiBuilder::new().layer_id(layer_id),
-            );
-
-            ui.set_opacity(0.7);
-
-            let response = drag_pill_frame()
-                .show(&mut ui, |ui| {
-                    let text_color = ui.visuals().widgets.inactive.text_color();
-
-                    ui.horizontal(|ui| {
-                        ui.spacing_mut().item_spacing.x = 2.0;
-
-                        ui.small_icon(icon, Some(text_color));
-                        ui.label(egui::RichText::new(payload.to_string()).color(text_color));
-                    });
-                })
-                .response;
-
-            let delta = pointer_pos - response.rect.right_bottom();
-            ctx.transform_layer_shapes(layer_id, emath::TSTransform::from_translation(delta));
+/// This helper should be constructed at the beginning of the frame and disposed of at the end.
+/// Its [`Self::payload_cursor_ui`] method should be called late during the frame (after the rest of
+/// the UI has a chance to update the feedback).
+pub struct DragAndDropManager {
+    /// Items that may not be dragged, e.g., because they are not movable nor copiable.
+    undraggable_items: ItemCollection,
+
+    feedback: Arc<Mutex<DragAndDropFeedback>>,
+}
+
+impl DragAndDropManager {
+    /// Create a [`DragAndDropManager`] by providing a list of undraggable items.
+    pub fn new(undraggable_items: impl Into<ItemCollection>) -> Self {
+        Self {
+            undraggable_items: undraggable_items.into(),
+            feedback: Default::default(),
+        }
+    }
+
+    /// Set the feedback to display to the user based on drop acceptability for the UI currently
+    /// hovered.
+    ///
+    /// By default, the feedback is unset and the pill/cursor are displayed in a "neutral" way,
+    /// indicating that the current drag-and-drop payload is valid but not hovered over a related
+    /// UI.
+    ///
+    /// If the payload type is compatible with the hovered UI element, that element should set the
+    /// feedback to either [`DragAndDropFeedback::Accept`] or [`DragAndDropFeedback::Reject`], based
+    /// on whether the actual payload content may meaningfully be dropped.
+    ///
+    /// For example, a view generally accepts a dragged entity but may occasionally reject it if
+    /// it already contains it.
+    pub fn set_feedback(&self, feedback: DragAndDropFeedback) {
+        *self.feedback.lock() = feedback;
+    }
+
+    /// Checks if items are draggable based on the list of undraggable items.
+    pub fn are_items_draggable(&self, items: &ItemCollection) -> bool {
+        self.undraggable_items
+            .iter_items()
+            .all(|item| !items.contains_item(item))
+    }
+
+    /// Display the currently dragged payload as a pill in the UI.
+    ///
+    /// This should be called once per frame.
+    pub fn payload_cursor_ui(&self, ctx: &egui::Context) {
+        if let Some(payload) = egui::DragAndDrop::payload::<DragAndDropPayload>(ctx) {
+            if let Some(pointer_pos) = ctx.pointer_interact_pos() {
+                let icon = match payload.as_ref() {
+                    DragAndDropPayload::Contents { .. } => &re_ui::icons::DND_MOVE,
+                    DragAndDropPayload::Entities { .. } => &re_ui::icons::DND_ADD_TO_EXISTING,
+                    // don't draw anything for invalid selection
+                    DragAndDropPayload::Invalid => return,
+                };
+
+                let layer_id = egui::LayerId::new(
+                    egui::Order::Tooltip,
+                    egui::Id::new("drag_and_drop_payload_layer"),
+                );
+
+                let mut ui = egui::Ui::new(
+                    ctx.clone(),
+                    egui::Id::new("rerun_drag_and_drop_payload_ui"),
+                    egui::UiBuilder::new().layer_id(layer_id),
+                );
+
+                ui.set_opacity(0.7);
+
+                //TODO: we should handle all 3 states differently.
+                let payload_is_currently_droppable =
+                    *self.feedback.lock() == DragAndDropFeedback::Accept;
+                let response = drag_pill_frame(payload_is_currently_droppable)
+                    .show(&mut ui, |ui| {
+                        let text_color = ui.visuals().widgets.inactive.text_color();
+
+                        ui.horizontal(|ui| {
+                            ui.spacing_mut().item_spacing.x = 2.0;
+
+                            ui.small_icon(icon, Some(text_color));
+                            ui.label(egui::RichText::new(payload.to_string()).color(text_color));
+                        });
+                    })
+                    .response;
+
+                let delta = pointer_pos - response.rect.right_bottom();
+                ctx.transform_layer_shapes(layer_id, emath::TSTransform::from_translation(delta));
+            }
         }
     }
 }
 
-fn drag_pill_frame() -> egui::Frame {
-    let hue = Hue::Blue;
+fn drag_pill_frame(droppable: bool) -> egui::Frame {
+    let hue = if droppable { Hue::Blue } else { Hue::Gray };
 
     egui::Frame {
         fill: re_ui::design_tokens().color(ColorToken::new(hue, S325)),
@@ -134,7 +240,7 @@ fn drag_pill_frame() -> egui::Frame {
             1.0,
             re_ui::design_tokens().color(ColorToken::new(hue, S375)),
         ),
-        rounding: (2.0).into(),
+        rounding: 2.0.into(),
         inner_margin: egui::Margin {
             left: 6.0,
             right: 9.0,
diff --git a/crates/viewer/re_viewer_context/src/lib.rs b/crates/viewer/re_viewer_context/src/lib.rs
index d276cb4b793b..c915efe53fba 100644
--- a/crates/viewer/re_viewer_context/src/lib.rs
+++ b/crates/viewer/re_viewer_context/src/lib.rs
@@ -53,7 +53,7 @@ pub use self::{
     component_ui_registry::{ComponentUiRegistry, ComponentUiTypes, UiLayout},
     contents::{blueprint_id_to_tile_id, Contents, ContentsName},
     data_result_node_or_path::DataResultNodeOrPath,
-    drag_and_drop::{drag_and_drop_payload_cursor_ui, DragAndDropPayload},
+    drag_and_drop::{DragAndDropFeedback, DragAndDropManager, DragAndDropPayload},
     file_dialog::santitize_file_name,
     image_info::{ColormapWithRange, ImageInfo},
     item::Item,
diff --git a/crates/viewer/re_viewer_context/src/test_context.rs b/crates/viewer/re_viewer_context/src/test_context.rs
index 70fae8e9238e..0e2cbc4b829f 100644
--- a/crates/viewer/re_viewer_context/src/test_context.rs
+++ b/crates/viewer/re_viewer_context/src/test_context.rs
@@ -6,8 +6,8 @@ use re_log_types::{StoreId, StoreKind};
 
 use crate::{
     blueprint_timeline, command_channel, ApplicationSelectionState, CommandReceiver, CommandSender,
-    ComponentUiRegistry, RecordingConfig, StoreContext, SystemCommand, ViewClassRegistry,
-    ViewerContext,
+    ComponentUiRegistry, ItemCollection, RecordingConfig, StoreContext, SystemCommand,
+    ViewClassRegistry, ViewerContext,
 };
 
 /// Harness to execute code that rely on [`crate::ViewerContext`].
@@ -90,7 +90,7 @@ impl TestContext {
             hub: &Default::default(),
         };
 
-        let undraggable_items = Default::default();
+        let drag_and_drop_helper = crate::DragAndDropManager::new(ItemCollection::default());
 
         let ctx = ViewerContext {
             app_options: &Default::default(),
@@ -110,7 +110,7 @@ impl TestContext {
             render_ctx: None,
             command_sender: &self.command_sender,
             focused_item: &None,
-            undraggable_items: &undraggable_items,
+            drag_and_drop_manager: &drag_and_drop_helper,
         };
 
         func(&ctx);
diff --git a/crates/viewer/re_viewer_context/src/viewer_context.rs b/crates/viewer/re_viewer_context/src/viewer_context.rs
index 1b1d470dc7c5..1728587c7aaf 100644
--- a/crates/viewer/re_viewer_context/src/viewer_context.rs
+++ b/crates/viewer/re_viewer_context/src/viewer_context.rs
@@ -8,8 +8,9 @@ use re_query::StorageEngineReadGuard;
 use crate::drag_and_drop::DragAndDropPayload;
 use crate::{
     query_context::DataQueryResult, AppOptions, ApplicableEntities, ApplicationSelectionState,
-    Caches, CommandSender, ComponentUiRegistry, IndicatedEntities, ItemCollection, PerVisualizer,
-    StoreContext, SystemCommandSender as _, TimeControl, ViewClassRegistry, ViewId,
+    Caches, CommandSender, ComponentUiRegistry, DragAndDropManager, IndicatedEntities,
+    ItemCollection, PerVisualizer, StoreContext, SystemCommandSender as _, TimeControl,
+    ViewClassRegistry, ViewId,
 };
 
 /// Common things needed by many parts of the viewer.
@@ -81,12 +82,8 @@ pub struct ViewerContext<'a> {
     /// that last several frames.
     pub focused_item: &'a Option<crate::Item>,
 
-    /// If a selection contains any `undraggable_items`, it may not be dragged.
-    ///
-    /// This is a rather ugly workaround to handle the case of the root container not being
-    /// draggable, but also being unknown to the drag-and-drop machinery in `re_viewer_context`.
-    //TODO(ab): figure out a way to deal with that in a cleaner way.
-    pub undraggable_items: &'a ItemCollection,
+    /// Helper object to manage drag-and-drop operations.
+    pub drag_and_drop_manager: &'a DragAndDropManager,
 }
 
 impl ViewerContext<'_> {
@@ -180,9 +177,8 @@ impl ViewerContext<'_> {
             }
 
             let selection_may_be_dragged = self
-                .undraggable_items
-                .iter_items()
-                .all(|item| !selected_items.contains_item(item));
+                .drag_and_drop_manager
+                .are_items_draggable(&selected_items);
 
             let payload = if selection_may_be_dragged {
                 DragAndDropPayload::from_items(&selected_items)
diff --git a/crates/viewer/re_viewport/src/viewport_ui.rs b/crates/viewer/re_viewport/src/viewport_ui.rs
index 804f3e14a24e..3dd62c5f3573 100644
--- a/crates/viewer/re_viewport/src/viewport_ui.rs
+++ b/crates/viewer/re_viewport/src/viewport_ui.rs
@@ -9,9 +9,9 @@ use re_context_menu::{context_menu_ui_for_item, SelectionUpdateBehavior};
 use re_log_types::{EntityPath, EntityPathRule};
 use re_ui::{design_tokens, ContextExt as _, DesignTokens, Icon, UiExt as _};
 use re_viewer_context::{
-    blueprint_id_to_tile_id, icon_for_container_kind, Contents, DragAndDropPayload, Item,
-    PublishedViewInfo, SystemExecutionOutput, ViewClassRegistry, ViewId, ViewQuery, ViewStates,
-    ViewerContext,
+    blueprint_id_to_tile_id, icon_for_container_kind, Contents, DragAndDropFeedback,
+    DragAndDropPayload, Item, PublishedViewInfo, SystemExecutionOutput, ViewClassRegistry, ViewId,
+    ViewQuery, ViewStates, ViewerContext,
 };
 use re_viewport_blueprint::{
     create_entity_add_info, ViewBlueprint, ViewportBlueprint, ViewportCommand,
@@ -254,6 +254,14 @@ impl ViewportUi {
         };
 
         let any_is_visualizable = entities.iter().any(can_entity_be_added);
+
+        ctx.drag_and_drop_manager
+            .set_feedback(if any_is_visualizable {
+                DragAndDropFeedback::Accept
+            } else {
+                DragAndDropFeedback::Reject
+            });
+
         if !any_is_visualizable {
             return false;
         }

From 5638e79ec7370aa2ec3b98bca620b38d83a14f89 Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Thu, 12 Dec 2024 21:29:38 +0100
Subject: [PATCH 11/16] Update cursor based on feedback from drop zone

---
 Cargo.lock                                    | 22 +++++++++----------
 Cargo.toml                                    | 14 ++++++------
 .../re_viewer_context/src/drag_and_drop.rs    | 13 ++++++++---
 3 files changed, 28 insertions(+), 21 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index bd21dfd45a73..7bec7c9064de 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1934,7 +1934,7 @@ checksum = "0d6ef0072f8a535281e4876be788938b528e9a1d43900b82c2569af7da799125"
 [[package]]
 name = "ecolor"
 version = "0.29.1"
-source = "git+https://github.com/emilk/egui.git?rev=13352d606496d7b1c5fd6fcfbe3c85baae39c040#13352d606496d7b1c5fd6fcfbe3c85baae39c040"
+source = "git+https://github.com/emilk/egui.git?rev=ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048#ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048"
 dependencies = [
  "bytemuck",
  "color-hex",
@@ -1951,7 +1951,7 @@ checksum = "18aade80d5e09429040243ce1143ddc08a92d7a22820ac512610410a4dd5214f"
 [[package]]
 name = "eframe"
 version = "0.29.1"
-source = "git+https://github.com/emilk/egui.git?rev=13352d606496d7b1c5fd6fcfbe3c85baae39c040#13352d606496d7b1c5fd6fcfbe3c85baae39c040"
+source = "git+https://github.com/emilk/egui.git?rev=ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048#ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048"
 dependencies = [
  "ahash",
  "bytemuck",
@@ -1990,7 +1990,7 @@ dependencies = [
 [[package]]
 name = "egui"
 version = "0.29.1"
-source = "git+https://github.com/emilk/egui.git?rev=13352d606496d7b1c5fd6fcfbe3c85baae39c040#13352d606496d7b1c5fd6fcfbe3c85baae39c040"
+source = "git+https://github.com/emilk/egui.git?rev=ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048#ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048"
 dependencies = [
  "accesskit",
  "ahash",
@@ -2007,7 +2007,7 @@ dependencies = [
 [[package]]
 name = "egui-wgpu"
 version = "0.29.1"
-source = "git+https://github.com/emilk/egui.git?rev=13352d606496d7b1c5fd6fcfbe3c85baae39c040#13352d606496d7b1c5fd6fcfbe3c85baae39c040"
+source = "git+https://github.com/emilk/egui.git?rev=ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048#ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048"
 dependencies = [
  "ahash",
  "bytemuck",
@@ -2026,7 +2026,7 @@ dependencies = [
 [[package]]
 name = "egui-winit"
 version = "0.29.1"
-source = "git+https://github.com/emilk/egui.git?rev=13352d606496d7b1c5fd6fcfbe3c85baae39c040#13352d606496d7b1c5fd6fcfbe3c85baae39c040"
+source = "git+https://github.com/emilk/egui.git?rev=ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048#ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048"
 dependencies = [
  "accesskit_winit",
  "ahash",
@@ -2068,7 +2068,7 @@ dependencies = [
 [[package]]
 name = "egui_extras"
 version = "0.29.1"
-source = "git+https://github.com/emilk/egui.git?rev=13352d606496d7b1c5fd6fcfbe3c85baae39c040#13352d606496d7b1c5fd6fcfbe3c85baae39c040"
+source = "git+https://github.com/emilk/egui.git?rev=ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048#ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048"
 dependencies = [
  "ahash",
  "egui",
@@ -2085,7 +2085,7 @@ dependencies = [
 [[package]]
 name = "egui_glow"
 version = "0.29.1"
-source = "git+https://github.com/emilk/egui.git?rev=13352d606496d7b1c5fd6fcfbe3c85baae39c040#13352d606496d7b1c5fd6fcfbe3c85baae39c040"
+source = "git+https://github.com/emilk/egui.git?rev=ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048#ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048"
 dependencies = [
  "ahash",
  "bytemuck",
@@ -2103,7 +2103,7 @@ dependencies = [
 [[package]]
 name = "egui_kittest"
 version = "0.29.1"
-source = "git+https://github.com/emilk/egui.git?rev=13352d606496d7b1c5fd6fcfbe3c85baae39c040#13352d606496d7b1c5fd6fcfbe3c85baae39c040"
+source = "git+https://github.com/emilk/egui.git?rev=ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048#ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048"
 dependencies = [
  "dify",
  "egui",
@@ -2172,7 +2172,7 @@ checksum = "60b1af1c220855b6ceac025d3f6ecdd2b7c4894bfe9cd9bda4fbb4bc7c0d4cf0"
 [[package]]
 name = "emath"
 version = "0.29.1"
-source = "git+https://github.com/emilk/egui.git?rev=13352d606496d7b1c5fd6fcfbe3c85baae39c040#13352d606496d7b1c5fd6fcfbe3c85baae39c040"
+source = "git+https://github.com/emilk/egui.git?rev=ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048#ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048"
 dependencies = [
  "bytemuck",
  "serde",
@@ -2288,7 +2288,7 @@ dependencies = [
 [[package]]
 name = "epaint"
 version = "0.29.1"
-source = "git+https://github.com/emilk/egui.git?rev=13352d606496d7b1c5fd6fcfbe3c85baae39c040#13352d606496d7b1c5fd6fcfbe3c85baae39c040"
+source = "git+https://github.com/emilk/egui.git?rev=ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048#ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048"
 dependencies = [
  "ab_glyph",
  "ahash",
@@ -2307,7 +2307,7 @@ dependencies = [
 [[package]]
 name = "epaint_default_fonts"
 version = "0.29.1"
-source = "git+https://github.com/emilk/egui.git?rev=13352d606496d7b1c5fd6fcfbe3c85baae39c040#13352d606496d7b1c5fd6fcfbe3c85baae39c040"
+source = "git+https://github.com/emilk/egui.git?rev=ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048#ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048"
 
 [[package]]
 name = "equivalent"
diff --git a/Cargo.toml b/Cargo.toml
index 73d17d1b009f..1ac77b684f88 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -559,13 +559,13 @@ significant_drop_tightening = "allow" # An update of parking_lot made this trigg
 # As a last resport, patch with a commit to our own repository.
 # ALWAYS document what PR the commit hash is part of, or when it was merged into the upstream trunk.
 
-ecolor = { git = "https://github.com/emilk/egui.git", rev = "13352d606496d7b1c5fd6fcfbe3c85baae39c040" }       # egui master 2024-12-09
-eframe = { git = "https://github.com/emilk/egui.git", rev = "13352d606496d7b1c5fd6fcfbe3c85baae39c040" }       # egui master 2024-12-09
-egui = { git = "https://github.com/emilk/egui.git", rev = "13352d606496d7b1c5fd6fcfbe3c85baae39c040" }         # egui master 2024-12-09
-egui_extras = { git = "https://github.com/emilk/egui.git", rev = "13352d606496d7b1c5fd6fcfbe3c85baae39c040" }  # egui master 2024-12-09
-egui_kittest = { git = "https://github.com/emilk/egui.git", rev = "13352d606496d7b1c5fd6fcfbe3c85baae39c040" } # egui master 2024-12-09
-egui-wgpu = { git = "https://github.com/emilk/egui.git", rev = "13352d606496d7b1c5fd6fcfbe3c85baae39c040" }    # egui master 2024-12-09
-emath = { git = "https://github.com/emilk/egui.git", rev = "13352d606496d7b1c5fd6fcfbe3c85baae39c040" }        # egui master 2024-12-09
+ecolor = { git = "https://github.com/emilk/egui.git", rev = "ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048" }       # egui master 2024-12-12
+eframe = { git = "https://github.com/emilk/egui.git", rev = "ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048" }       # egui master 2024-12-12
+egui = { git = "https://github.com/emilk/egui.git", rev = "ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048" }         # egui master 2024-12-12
+egui_extras = { git = "https://github.com/emilk/egui.git", rev = "ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048" }  # egui master 2024-12-12
+egui_kittest = { git = "https://github.com/emilk/egui.git", rev = "ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048" } # egui master 2024-12-12
+egui-wgpu = { git = "https://github.com/emilk/egui.git", rev = "ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048" }    # egui master 2024-12-12
+emath = { git = "https://github.com/emilk/egui.git", rev = "ba060a2c878bfefa5aead8e5fe0b9e69ab4c9048" }        # egui master 2024-12-12
 
 # Useful while developing:
 # ecolor = { path = "../../egui/crates/ecolor" }
diff --git a/crates/viewer/re_viewer_context/src/drag_and_drop.rs b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
index 8c3f2553c64f..e0089ed643e5 100644
--- a/crates/viewer/re_viewer_context/src/drag_and_drop.rs
+++ b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
@@ -208,9 +208,16 @@ impl DragAndDropManager {
 
                 ui.set_opacity(0.7);
 
-                //TODO: we should handle all 3 states differently.
-                let payload_is_currently_droppable =
-                    *self.feedback.lock() == DragAndDropFeedback::Accept;
+                let feedback = *self.feedback.lock();
+
+                match feedback {
+                    DragAndDropFeedback::Accept | DragAndDropFeedback::Ignore => {
+                        ctx.set_cursor_icon(egui::CursorIcon::Grabbing)
+                    }
+                    DragAndDropFeedback::Reject => ctx.set_cursor_icon(egui::CursorIcon::NoDrop),
+                }
+
+                let payload_is_currently_droppable = feedback == DragAndDropFeedback::Accept;
                 let response = drag_pill_frame(payload_is_currently_droppable)
                     .show(&mut ui, |ui| {
                         let text_color = ui.visuals().widgets.inactive.text_color();

From 3738fc94bc2b2db1575ed2494611a2a5a31e6cfb Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Fri, 13 Dec 2024 10:36:34 +0100
Subject: [PATCH 12/16] Pill opacity 50/70% depending on feedback + fixed
 pilled being cropped

---
 .../re_viewer_context/src/drag_and_drop.rs    | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/crates/viewer/re_viewer_context/src/drag_and_drop.rs b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
index e0089ed643e5..314615c23c4a 100644
--- a/crates/viewer/re_viewer_context/src/drag_and_drop.rs
+++ b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
@@ -206,15 +206,22 @@ impl DragAndDropManager {
                     egui::UiBuilder::new().layer_id(layer_id),
                 );
 
-                ui.set_opacity(0.7);
-
                 let feedback = *self.feedback.lock();
 
                 match feedback {
-                    DragAndDropFeedback::Accept | DragAndDropFeedback::Ignore => {
-                        ctx.set_cursor_icon(egui::CursorIcon::Grabbing)
+                    DragAndDropFeedback::Accept => {
+                        ctx.set_cursor_icon(egui::CursorIcon::Grabbing);
+                        ui.set_opacity(0.8);
+                    }
+
+                    DragAndDropFeedback::Ignore => {
+                        ctx.set_cursor_icon(egui::CursorIcon::Grabbing);
+                        ui.set_opacity(0.5);
+                    }
+                    DragAndDropFeedback::Reject => {
+                        ctx.set_cursor_icon(egui::CursorIcon::NoDrop);
+                        ui.set_opacity(0.5);
                     }
-                    DragAndDropFeedback::Reject => ctx.set_cursor_icon(egui::CursorIcon::NoDrop),
                 }
 
                 let payload_is_currently_droppable = feedback == DragAndDropFeedback::Accept;
@@ -254,6 +261,8 @@ fn drag_pill_frame(droppable: bool) -> egui::Frame {
             top: 5.0,
             bottom: 4.0,
         },
+        //TODO(ab): needed to avoid the pill being cropped, not sure why?
+        outer_margin: egui::Margin::same(1.0),
         ..Default::default()
     }
 }

From 12b1a36de7c5eb68f236813d6bb4d8b09d7a585e Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Fri, 13 Dec 2024 11:10:08 +0100
Subject: [PATCH 13/16] Don't force select on drag

---
 .../re_viewer_context/src/viewer_context.rs   | 51 ++++++++++++-------
 1 file changed, 32 insertions(+), 19 deletions(-)

diff --git a/crates/viewer/re_viewer_context/src/viewer_context.rs b/crates/viewer/re_viewer_context/src/viewer_context.rs
index 1728587c7aaf..1ab2a88c1db7 100644
--- a/crates/viewer/re_viewer_context/src/viewer_context.rs
+++ b/crates/viewer/re_viewer_context/src/viewer_context.rs
@@ -144,59 +144,72 @@ impl ViewerContext<'_> {
     /// are some guidelines:
     /// - Is there a meaningful destination for the dragged payload? For example, dragging stuff out
     ///   of a modal dialog is by definition meaningless.
-    /// - Even if a drag destination exists, would that be obvious for the user?
+    /// - Even if a drag destination exists, would that be obvious to the user?
     /// - Is it expected for that kind of UI element to be draggable? For example, buttons aren't
     ///   typically draggable.
+    ///
+    /// Drag vs. selection semantics:
+    ///
+    /// - When dragging an unselected item, that item only is dragged, and the selection is
+    ///   unchanged…
+    /// - …unless cmd/ctrl is held, in which case the item is added to the selection and the entire
+    ///   selection is dragged.
+    /// - When dragging a selected item, the entire selection is dragged as well.
     pub fn handle_select_hover_drag_interactions(
         &self,
         response: &egui::Response,
-        selection: impl Into<ItemCollection>,
+        interacted_items: impl Into<ItemCollection>,
         draggable: bool,
     ) {
         re_tracing::profile_function!();
 
-        let selection = selection.into().into_mono_instance_path_items(self);
+        let interacted_items = interacted_items.into().into_mono_instance_path_items(self);
         let selection_state = self.selection_state();
 
         if response.hovered() {
-            selection_state.set_hovered(selection.clone());
+            selection_state.set_hovered(interacted_items.clone());
         }
 
         if draggable && response.drag_started() {
             let mut selected_items = selection_state.selected_items().clone();
-            let is_already_selected = selection
+            let is_already_selected = interacted_items
                 .iter()
                 .all(|(item, _)| selected_items.contains_item(item));
-            if !is_already_selected {
-                if response.ctx.input(|i| i.modifiers.command) {
-                    selected_items.extend(selection);
-                } else {
-                    selected_items = selection;
-                }
+
+            let is_cmd_held = response.ctx.input(|i| i.modifiers.command);
+
+            // see semantics description in the docstring
+            let dragged_items = if !is_already_selected && is_cmd_held {
+                selected_items.extend(interacted_items);
                 selection_state.set_selection(selected_items.clone());
-            }
+                selected_items
+            } else if !is_already_selected {
+                interacted_items
+            } else {
+                selected_items
+            };
 
-            let selection_may_be_dragged = self
+            let items_may_be_dragged = self
                 .drag_and_drop_manager
-                .are_items_draggable(&selected_items);
+                .are_items_draggable(&dragged_items);
 
-            let payload = if selection_may_be_dragged {
-                DragAndDropPayload::from_items(&selected_items)
+            let payload = if items_may_be_dragged {
+                DragAndDropPayload::from_items(&dragged_items)
             } else {
                 DragAndDropPayload::Invalid
             };
 
             egui::DragAndDrop::set_payload(&response.ctx, payload);
         } else if response.double_clicked() {
-            if let Some(item) = selection.first_item() {
+            if let Some(item) = interacted_items.first_item() {
                 self.command_sender
                     .send_system(crate::SystemCommand::SetFocus(item.clone()));
             }
         } else if response.clicked() {
             if response.ctx.input(|i| i.modifiers.command) {
-                selection_state.toggle_selection(selection);
+                selection_state.toggle_selection(interacted_items);
             } else {
-                selection_state.set_selection(selection);
+                selection_state.set_selection(interacted_items);
             }
         }
     }

From 8559af70e8cf58a9775dc96c18c6e1dad7be1bb9 Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Fri, 13 Dec 2024 11:34:29 +0100
Subject: [PATCH 14/16] doclink

---
 crates/viewer/re_viewer_context/src/drag_and_drop.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/crates/viewer/re_viewer_context/src/drag_and_drop.rs b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
index 314615c23c4a..1d24bf2e2f03 100644
--- a/crates/viewer/re_viewer_context/src/drag_and_drop.rs
+++ b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
@@ -27,7 +27,7 @@
 //!    visual feedback. This includes:
 //!    - Calling [`DragAndDropManager::set_feedback`] with the appropriate feedback.
 //!    - Drawing a frame around the target container with
-//!      [`re_ui::DesignToken::drop_target_container_stroke`].
+//!      [`re_ui::DesignTokens::drop_target_container_stroke`].
 //!    - Optionally provide more feedback, e.g., where exactly the payload will be inserted within
 //!      the container.
 //!

From f5a7f3db797dcfe0cc1343896cad5a8c0add5d92 Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Mon, 16 Dec 2024 14:20:39 +0100
Subject: [PATCH 15/16] Add two release check list test

---
 .../check_drag_and_drop_selection.py          | 78 +++++++++++++++
 .../check_entity_drag_and_drop.py             | 94 +++++++++++++++++++
 2 files changed, 172 insertions(+)
 create mode 100644 tests/python/release_checklist/check_drag_and_drop_selection.py
 create mode 100644 tests/python/release_checklist/check_entity_drag_and_drop.py

diff --git a/tests/python/release_checklist/check_drag_and_drop_selection.py b/tests/python/release_checklist/check_drag_and_drop_selection.py
new file mode 100644
index 000000000000..2ca4d7ee9b82
--- /dev/null
+++ b/tests/python/release_checklist/check_drag_and_drop_selection.py
@@ -0,0 +1,78 @@
+from __future__ import annotations
+
+import os
+from argparse import Namespace
+from uuid import uuid4
+
+import numpy as np
+import rerun as rr
+import rerun.blueprint as rrb
+
+README = """\
+# Drag-and-drop selection
+
+The goal of this test is to test the selection behavior of drag-and-drop.
+
+#### View selects on a successful drop
+
+1. Select the `cos_curve` entity in the streams tree.
+2. Drag it to the PLOT view and drop it.
+3. _Expect_: the entity is added to the view, and the view becomes selected.
+
+
+#### View doesn't select on a failed drop
+
+1. Select the `cos_curve` entity again.
+2. Drag it to the PLOT view (it should be rejected) and drop it.
+3. _Expect_: nothing happens, and the selection is not changed.
+
+
+#### Dragging an unselected item doesn't change the selection
+
+1. Select the PLOT view.
+2. Drag drag the `line_curve` entity to the PLOT view and drop it.
+2. _Expect_:
+    - The selection remains unchanged (the PLOT view is still selected).
+    - The `line_curve` entity is added to the view.
+
+"""
+
+
+def log_readme() -> None:
+    rr.log("readme", rr.TextDocument(README, media_type=rr.MediaType.MARKDOWN), static=True)
+
+
+def blueprint() -> rrb.BlueprintLike:
+    return rrb.Horizontal(
+        rrb.TimeSeriesView(origin="/", contents=[], name="PLOT"),
+        rrb.TextDocumentView(origin="readme"),
+    )
+
+
+def log_some_scalar_entities() -> None:
+    times = np.arange(100)
+    curves = [
+        ("cos_curve", np.cos(times / 100 * 2 * np.pi)),
+        ("line_curve", times / 100 + 0.2),
+    ]
+
+    time_column = rr.TimeSequenceColumn("frame", times)
+
+    for path, curve in curves:
+        rr.send_columns(path, times=[time_column], components=[rr.components.ScalarBatch(curve)])
+
+
+def run(args: Namespace) -> None:
+    rr.script_setup(args, f"{os.path.basename(__file__)}", recording_id=uuid4(), default_blueprint=blueprint())
+
+    log_readme()
+    log_some_scalar_entities()
+
+
+if __name__ == "__main__":
+    import argparse
+
+    parser = argparse.ArgumentParser(description="Interactive release checklist")
+    rr.script_add_args(parser)
+    args = parser.parse_args()
+    run(args)
diff --git a/tests/python/release_checklist/check_entity_drag_and_drop.py b/tests/python/release_checklist/check_entity_drag_and_drop.py
new file mode 100644
index 000000000000..0e9d718064dd
--- /dev/null
+++ b/tests/python/release_checklist/check_entity_drag_and_drop.py
@@ -0,0 +1,94 @@
+from __future__ import annotations
+
+import os
+from argparse import Namespace
+from uuid import uuid4
+
+import numpy as np
+import rerun as rr
+import rerun.blueprint as rrb
+
+README = """\
+# Entity drag-and-drop
+
+The goal of this test is to check the behavior of views when entities are dragged-and-dropped over them.
+The drag payload pill _and_ the mouse cursor should change based on the underlying UI element feedback.
+This table summarizes the three possible states:
+
+**Note**: actual cursor shape may vary depending on the platform (OS, browser, etc).
+
+| **Ignore** | **Accept** | **Reject** |
+| --- | --- | --- |
+| Gray pill | Blue pill | Gray pill |
+| Hand cursor | Hand cursor | No-drop cursor |
+| ![ignore](https://static.rerun.io/dnd-cursor-ignore/ec48f64a119bddd2c9cbd55410021ef0e1a30feb/full.png) | ![accept](https://static.rerun.io/dnd-cursor-accept/7b40cd79fd99ba2c31617d2f40f56c5c8ba3aca0/full.png) | ![reject](https://static.rerun.io/dnd-cursor-reject/6f105e9689be33b2e0fff5bb1ad42cbb6271b622/full.png) |
+
+
+#### Ignore state
+
+1. Drag any entity from the streams tree over the blueprint tree (which ignores entities).
+2. _Expect_: pill/cursor in ignore state, nothing happens on drop
+
+
+#### Accept state
+
+1. Drag the `cos_curve` entity from the streams tree to the BOTTOM view and drop it.
+2. _Expect_: pill/cursor in accept state, entity is added to the BOTTOM view on drop.
+
+
+#### Reject state
+
+1. Drag THE SAME `cos_curve` entity from the streams tree.
+2. _Expect_:
+    - BOTTOM rejects the entity, nothing happens on drop.
+    - TOP accepts the entity, entity is added to the TOP view on drop.
+
+#### Multi-selection drag
+
+1. Multi-select `cos_curve` and `line_curve` entities from the streams tree.
+2. _Expect_: both views accept the entities, only `line_curve` is added on drop.
+
+"""
+
+
+def log_readme() -> None:
+    rr.log("readme", rr.TextDocument(README, media_type=rr.MediaType.MARKDOWN), static=True)
+
+
+def blueprint() -> rrb.BlueprintLike:
+    return rrb.Horizontal(
+        rrb.Vertical(
+            rrb.TimeSeriesView(origin="/", contents=[], name="TOP"),
+            rrb.TimeSeriesView(origin="/", contents=[], name="BOTTOM"),
+        ),
+        rrb.TextDocumentView(origin="readme"),
+    )
+
+
+def log_some_scalar_entities() -> None:
+    times = np.arange(100)
+    curves = [
+        ("cos_curve", np.cos(times / 100 * 2 * np.pi)),
+        ("line_curve", times / 100 + 0.2),
+    ]
+
+    time_column = rr.TimeSequenceColumn("frame", times)
+
+    for path, curve in curves:
+        rr.send_columns(path, times=[time_column], components=[rr.components.ScalarBatch(curve)])
+
+
+def run(args: Namespace) -> None:
+    rr.script_setup(args, f"{os.path.basename(__file__)}", recording_id=uuid4(), default_blueprint=blueprint())
+
+    log_readme()
+    log_some_scalar_entities()
+
+
+if __name__ == "__main__":
+    import argparse
+
+    parser = argparse.ArgumentParser(description="Interactive release checklist")
+    rr.script_add_args(parser)
+    args = parser.parse_args()
+    run(args)

From a2716893750c6e34b59492844e3d04446bbadf9f Mon Sep 17 00:00:00 2001
From: Antoine Beyeler <antoine@rerun.io>
Date: Tue, 17 Dec 2024 10:53:20 +0100
Subject: [PATCH 16/16] Review comment

---
 Cargo.lock                                           |  1 +
 crates/viewer/re_viewer_context/Cargo.toml           |  1 +
 crates/viewer/re_viewer_context/src/drag_and_drop.rs | 12 ++++++------
 crates/viewer/re_viewer_context/src/test_context.rs  |  4 ++--
 4 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 1dba7ac4c891..8d679e2c1802 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -6851,6 +6851,7 @@ dependencies = [
  "bit-vec",
  "bitflags 2.6.0",
  "bytemuck",
+ "crossbeam",
  "directories",
  "egui",
  "egui-wgpu",
diff --git a/crates/viewer/re_viewer_context/Cargo.toml b/crates/viewer/re_viewer_context/Cargo.toml
index 71bd3774d201..9fc29fa4b27f 100644
--- a/crates/viewer/re_viewer_context/Cargo.toml
+++ b/crates/viewer/re_viewer_context/Cargo.toml
@@ -42,6 +42,7 @@ anyhow.workspace = true
 bit-vec.workspace = true
 bitflags.workspace = true
 bytemuck.workspace = true
+crossbeam.workspace = true
 directories.workspace = true
 egui_extras.workspace = true
 egui_tiles.workspace = true
diff --git a/crates/viewer/re_viewer_context/src/drag_and_drop.rs b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
index 1d24bf2e2f03..b7755fef0989 100644
--- a/crates/viewer/re_viewer_context/src/drag_and_drop.rs
+++ b/crates/viewer/re_viewer_context/src/drag_and_drop.rs
@@ -36,10 +36,8 @@
 //!    called.
 
 use std::fmt::{Display, Formatter};
-use std::sync::Arc;
 
 use itertools::Itertools;
-use parking_lot::Mutex;
 
 use re_entity_db::InstancePath;
 use re_log_types::EntityPath;
@@ -82,6 +80,8 @@ fn try_item_collection_to_contents(items: &ItemCollection) -> Option<Vec<Content
 fn try_item_collection_to_entities(items: &ItemCollection) -> Option<Vec<EntityPath>> {
     items
         .iter()
+        // Note: this is not a filter map, because we rely on the implicit "all" semantics of
+        // `collect`: we return `Some<Vec<_>>` only if all iterated items are `Some<_>`.
         .map(|(item, _)| match item {
             Item::InstancePath(instance_path) | Item::DataResult(_, instance_path) => instance_path
                 .is_all()
@@ -139,14 +139,14 @@ pub enum DragAndDropFeedback {
 
 /// Helper to handle drag-and-drop operations.
 ///
-/// This helper should be constructed at the beginning of the frame and disposed of at the end.
+/// This helper must be constructed at the beginning of the frame and disposed of at the end.
 /// Its [`Self::payload_cursor_ui`] method should be called late during the frame (after the rest of
 /// the UI has a chance to update the feedback).
 pub struct DragAndDropManager {
     /// Items that may not be dragged, e.g., because they are not movable nor copiable.
     undraggable_items: ItemCollection,
 
-    feedback: Arc<Mutex<DragAndDropFeedback>>,
+    feedback: crossbeam::atomic::AtomicCell<DragAndDropFeedback>,
 }
 
 impl DragAndDropManager {
@@ -172,7 +172,7 @@ impl DragAndDropManager {
     /// For example, a view generally accepts a dragged entity but may occasionally reject it if
     /// it already contains it.
     pub fn set_feedback(&self, feedback: DragAndDropFeedback) {
-        *self.feedback.lock() = feedback;
+        self.feedback.store(feedback);
     }
 
     /// Checks if items are draggable based on the list of undraggable items.
@@ -206,7 +206,7 @@ impl DragAndDropManager {
                     egui::UiBuilder::new().layer_id(layer_id),
                 );
 
-                let feedback = *self.feedback.lock();
+                let feedback = self.feedback.load();
 
                 match feedback {
                     DragAndDropFeedback::Accept => {
diff --git a/crates/viewer/re_viewer_context/src/test_context.rs b/crates/viewer/re_viewer_context/src/test_context.rs
index 0e2cbc4b829f..e661e60cf4fc 100644
--- a/crates/viewer/re_viewer_context/src/test_context.rs
+++ b/crates/viewer/re_viewer_context/src/test_context.rs
@@ -90,7 +90,7 @@ impl TestContext {
             hub: &Default::default(),
         };
 
-        let drag_and_drop_helper = crate::DragAndDropManager::new(ItemCollection::default());
+        let drag_and_drop_manager = crate::DragAndDropManager::new(ItemCollection::default());
 
         let ctx = ViewerContext {
             app_options: &Default::default(),
@@ -110,7 +110,7 @@ impl TestContext {
             render_ctx: None,
             command_sender: &self.command_sender,
             focused_item: &None,
-            drag_and_drop_manager: &drag_and_drop_helper,
+            drag_and_drop_manager: &drag_and_drop_manager,
         };
 
         func(&ctx);