Add UiBuilder::layer_id, and remove layer_id from Ui::new

crates/egui/src/containers/area.rs

@@ -540,6 +540,7 @@ impl Prepared {
 
         let mut ui_builder = UiBuilder::new()
             .ui_stack_info(UiStackInfo::new(self.kind))
+            .layer_id(self.layer_id)
             .max_rect(max_rect);
 
         if !self.enabled {
@@ -549,7 +550,7 @@ impl Prepared {
             ui_builder = ui_builder.sizing_pass().invisible();
         }
 
-        let mut ui = Ui::new(ctx.clone(), self.layer_id, self.layer_id.id, ui_builder);
+        let mut ui = Ui::new(ctx.clone(), self.layer_id.id, ui_builder);
         ui.set_clip_rect(self.constrain_rect); // Don't paint outside our bounds
 
         if self.fade_in {

crates/egui/src/containers/panel.rs

@@ -375,14 +375,14 @@ impl SidePanel {
         ctx: &Context,
         add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
     ) -> InnerResponse<R> {
-        let layer_id = LayerId::background();
         let side = self.side;
         let available_rect = ctx.available_rect();
         let mut panel_ui = Ui::new(
             ctx.clone(),
-            layer_id,
             self.id,
-            UiBuilder::new().max_rect(available_rect),
+            UiBuilder::new()
+                .layer_id(LayerId::background())
+                .max_rect(available_rect),
         );
         panel_ui.set_clip_rect(ctx.screen_rect());
 
@@ -868,15 +868,15 @@ impl TopBottomPanel {
         ctx: &Context,
         add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
     ) -> InnerResponse<R> {
-        let layer_id = LayerId::background();
         let available_rect = ctx.available_rect();
         let side = self.side;
 
         let mut panel_ui = Ui::new(
             ctx.clone(),
-            layer_id,
             self.id,
-            UiBuilder::new().max_rect(available_rect),
+            UiBuilder::new()
+                .layer_id(LayerId::background())
+                .max_rect(available_rect),
         );
         panel_ui.set_clip_rect(ctx.screen_rect());
 
@@ -1135,14 +1135,14 @@ impl CentralPanel {
         add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
     ) -> InnerResponse<R> {
         let available_rect = ctx.available_rect();
-        let layer_id = LayerId::background();
         let id = Id::new((ctx.viewport_id(), "central_panel"));
 
         let mut panel_ui = Ui::new(
             ctx.clone(),
-            layer_id,
             id,
-            UiBuilder::new().max_rect(available_rect),
+            UiBuilder::new()
+                .layer_id(LayerId::background())
+                .max_rect(available_rect),
         );
         panel_ui.set_clip_rect(ctx.screen_rect());
 

crates/egui/src/painter.rs

@@ -74,6 +74,9 @@ impl Painter {
     }
 
     /// Redirect where you are painting.
+    ///
+    /// It is undefined behavior to change the [`LayerId`]
+    /// of [`crate::Ui::painter`].
     pub fn set_layer_id(&mut self, layer_id: LayerId) {
         self.layer_id = layer_id;
     }

crates/egui/src/ui.rs

@@ -120,10 +120,11 @@ impl Ui {
     ///
     /// Normally you would not use this directly, but instead use
     /// [`crate::SidePanel`], [`crate::TopBottomPanel`], [`crate::CentralPanel`], [`crate::Window`] or [`crate::Area`].
-    pub fn new(ctx: Context, layer_id: LayerId, id: Id, ui_builder: UiBuilder) -> Self {
+    pub fn new(ctx: Context, id: Id, ui_builder: UiBuilder) -> Self {
         let UiBuilder {
             id_salt,
             ui_stack_info,
+            layer_id,
             max_rect,
             layout,
             disabled,
@@ -133,6 +134,8 @@ impl Ui {
             sense,
         } = ui_builder;
 
+        let layer_id = layer_id.unwrap_or(LayerId::background());
+
         debug_assert!(
             id_salt.is_none(),
             "Top-level Ui:s should not have an id_salt"
@@ -170,7 +173,7 @@ impl Ui {
         };
 
         // Register in the widget stack early, to ensure we are behind all widgets we contain:
-        let start_rect = Rect::NOTHING; // This will be overwritten when/if `remember_min_rect` is called
+        let start_rect = Rect::NOTHING; // This will be overwritten when `remember_min_rect` is called
         ui.ctx().create_widget(
             WidgetRect {
                 id: ui.unique_id,
@@ -247,6 +250,7 @@ impl Ui {
         let UiBuilder {
             id_salt,
             ui_stack_info,
+            layer_id,
             max_rect,
             layout,
             disabled,
@@ -262,6 +266,9 @@ impl Ui {
         let max_rect = max_rect.unwrap_or_else(|| self.available_rect_before_wrap());
         let mut layout = layout.unwrap_or(*self.layout());
         let enabled = self.enabled && !disabled && !invisible;
+        if let Some(layer_id) = layer_id {
+            painter.set_layer_id(layer_id);
+        }
         if invisible {
             painter.set_invisible();
         }
@@ -310,7 +317,7 @@ impl Ui {
         };
 
         // Register in the widget stack early, to ensure we are behind all widgets we contain:
-        let start_rect = Rect::NOTHING; // This will be overwritten when/if `remember_min_rect` is called
+        let start_rect = Rect::NOTHING; // This will be overwritten when `remember_min_rect` is called
         child_ui.ctx().create_widget(
             WidgetRect {
                 id: child_ui.unique_id,
@@ -2306,15 +2313,13 @@ impl Ui {
     /// });
     /// # });
     /// ```
+    #[deprecated = "Use ui.scope_builder(UiBuilder::new().layer_id(…), …) instead"]
     pub fn with_layer_id<R>(
         &mut self,
         layer_id: LayerId,
         add_contents: impl FnOnce(&mut Self) -> R,
     ) -> InnerResponse<R> {
-        self.scope(|ui| {
-            ui.painter.set_layer_id(layer_id);
-            add_contents(ui)
-        })
+        self.scope_builder(UiBuilder::new().layer_id(layer_id), add_contents)
     }
 
     /// A [`CollapsingHeader`] that starts out collapsed.
@@ -2760,7 +2765,8 @@ impl Ui {
 
             // Paint the body to a new layer:
             let layer_id = LayerId::new(Order::Tooltip, id);
-            let InnerResponse { inner, response } = self.with_layer_id(layer_id, add_contents);
+            let InnerResponse { inner, response } =
+                self.scope_builder(UiBuilder::new().layer_id(layer_id), add_contents);
 
             // Now we move the visuals of the body to where the mouse is.
             // Normally you need to decide a location for a widget first,

crates/egui/src/ui_builder.rs

@@ -1,6 +1,6 @@
 use std::{hash::Hash, sync::Arc};
 
-use crate::{Id, Layout, Rect, Sense, Style, UiStackInfo};
+use crate::{Id, LayerId, Layout, Rect, Sense, Style, UiStackInfo};
 
 #[allow(unused_imports)] // Used for doclinks
 use crate::Ui;
@@ -15,6 +15,7 @@ use crate::Ui;
 pub struct UiBuilder {
     pub id_salt: Option<Id>,
     pub ui_stack_info: UiStackInfo,
+    pub layer_id: Option<LayerId>,
     pub max_rect: Option<Rect>,
     pub layout: Option<Layout>,
     pub disabled: bool,
@@ -48,6 +49,13 @@ impl UiBuilder {
         self
     }
 
+    /// Show the [`Ui`] in a different [`LayerId`] from its parent.
+    #[inline]
+    pub fn layer_id(mut self, layer_id: LayerId) -> Self {
+        self.layer_id = Some(layer_id);
+        self
+    }
+
     /// Set the max rectangle, within which widgets will go.
     ///
     /// New widgets will *try* to fit within this rectangle.

crates/egui/src/widget_rect.rs

@@ -139,6 +139,14 @@ impl WidgetRects {
                 // e.g. calling `response.interact(…)` to add more interaction.
                 let (idx_in_layer, existing) = entry.get_mut();
 
+                debug_assert!(
+                    existing.layer_id == widget_rect.layer_id,
+                    "Widget {:?} changed layer_id during the frame from {:?} to {:?}",
+                    widget_rect.id,
+                    existing.layer_id,
+                    widget_rect.layer_id
+                );
+
                 // Update it:
                 existing.rect = widget_rect.rect; // last wins
                 existing.interact_rect = widget_rect.interact_rect; // last wins

tests/test_viewports/src/main.rs

@@ -433,7 +433,7 @@ fn drag_source<R>(
 
         // Paint the body to a new layer:
         let layer_id = egui::LayerId::new(egui::Order::Tooltip, id);
-        let res = ui.with_layer_id(layer_id, body);
+        let res = ui.scope_builder(UiBuilder::new().layer_id(layer_id), body);
 
         if let Some(pointer_pos) = ui.ctx().pointer_interact_pos() {
             let delta = pointer_pos - res.response.rect.center();