Fix scroll_with_delta behaviour being inverted

crates/egui/src/containers/scroll_area.rs

@@ -787,7 +787,8 @@ impl Prepared {
             .frame_state_mut(|state| std::mem::take(&mut state.scroll_delta));
 
         for d in 0..2 {
-            let mut delta = scroll_delta[d];
+            // FrameState::scroll_delta is inverted from the way we apply the delta, so we need to negate it.
+            let mut delta = -scroll_delta[d];
 
             // We always take both scroll targets regardless of which scroll axes are enabled. This
             // is to avoid them leaking to other scroll areas.

crates/egui/src/frame_state.rs

@@ -42,6 +42,14 @@ pub(crate) struct FrameState {
     pub(crate) scroll_target: [Option<(Rangef, Option<Align>)>; 2],
 
     /// The current scroll area should scroll by this much.
+    ///
+    /// The delta dictates how the _content_ should move.
+    ///
+    /// A positive X-value indicates the content is being moved right,
+    /// as when swiping right on a touch-screen or track-pad with natural scrolling.
+    ///
+    /// A positive Y-value indicates the content is being moved down,
+    /// as when swiping down on a touch-screen or track-pad with natural scrolling.
     pub(crate) scroll_delta: Vec2,
 
     #[cfg(feature = "accesskit")]

crates/egui/src/ui.rs

@@ -1073,7 +1073,7 @@ impl Ui {
     /// A positive Y-value indicates the content is being moved down,
     /// as when swiping down on a touch-screen or track-pad with natural scrolling.
     ///
-    /// If this is called multiple times per frame for the same ScrollArea, the deltas will be combined.
+    /// If this is called multiple times per frame for the same [`ScrollArea`], the deltas will be combined.
     ///
     /// /// See also: [`Response::scroll_to_me`], [`Ui::scroll_to_rect`], [`Ui::scroll_to_cursor`]
     ///

crates/egui_demo_lib/src/demo/scrolling.rs

@@ -294,7 +294,7 @@ impl super::View for ScrollTo {
             scroll_top |= ui.button("Scroll to top").clicked();
             scroll_bottom |= ui.button("Scroll to bottom").clicked();
             if ui.button("Scroll down by 64px").clicked() {
-                scroll_delta = Some(Vec2::new(0.0, 64.0));
+                scroll_delta = Some(Vec2::new(0.0, -64.0));
             }
         });