ScrollPosition scale factor fix (#16617)

# Objective

Scroll position uses physical coordinates. This means scrolling may go
faster or slower depending on the scroll factor. Also the scrolled
position will change when the scale factor changes.

## Solution

In `ui_layout_system` convert `max_possible_offset` to logical
coordinates before clamping the scroll position. Then convert the
clamped scroll position to physical coordinates before propagating it to
the node's children.

## Testing

Look at the `scroll` example. On main if you change your display's scale
factor the items displayed by the scrolling lists will change because
`ScrollPosition`'s displacement values don't respect scale factor. With
this PR the displacement will be scaled too, and the won't move.

Author: ickshonpe

crates/bevy_ui/src/layout/mod.rs

@@ -433,14 +433,20 @@ with UI components as a child of an entity without UI components, your UI layout
 
             let content_size = Vec2::new(layout.content_size.width, layout.content_size.height);
             let max_possible_offset = (content_size - layout_size).max(Vec2::ZERO);
-            let clamped_scroll_position = scroll_position.clamp(Vec2::ZERO, max_possible_offset);
+            let clamped_scroll_position = scroll_position.clamp(
+                Vec2::ZERO,
+                max_possible_offset * inverse_target_scale_factor,
+            );
 
             if clamped_scroll_position != scroll_position {
                 commands
                     .entity(entity)
-                    .insert(ScrollPosition::from(&clamped_scroll_position));
+                    .insert(ScrollPosition::from(clamped_scroll_position));
             }
 
+            let physical_scroll_position =
+                (clamped_scroll_position / inverse_target_scale_factor).round();
+
             for child_uinode in ui_children.iter_ui_children(entity) {
                 update_uinode_geometry_recursive(
                     commands,
@@ -451,7 +457,7 @@ with UI components as a child of an entity without UI components, your UI layout
                     ui_children,
                     inverse_target_scale_factor,
                     layout_size,
-                    clamped_scroll_position,
+                    physical_scroll_position,
                 );
             }
         }

crates/bevy_ui/src/ui_node.rs

@@ -237,9 +237,9 @@ impl Default for ComputedNode {
 #[derive(Component, Debug, Clone, Reflect)]
 #[reflect(Component, Default)]
 pub struct ScrollPosition {
-    /// How far across the node is scrolled, in pixels. (0 = not scrolled / scrolled to right)
+    /// How far across the node is scrolled, in logical pixels. (0 = not scrolled / scrolled to right)
     pub offset_x: f32,
-    /// How far down the node is scrolled, in pixels. (0 = not scrolled / scrolled to top)
+    /// How far down the node is scrolled, in logical pixels. (0 = not scrolled / scrolled to top)
     pub offset_y: f32,
 }
 
@@ -262,8 +262,8 @@ impl From<&ScrollPosition> for Vec2 {
     }
 }
 
-impl From<&Vec2> for ScrollPosition {
-    fn from(vec: &Vec2) -> Self {
+impl From<Vec2> for ScrollPosition {
+    fn from(vec: Vec2) -> Self {
         ScrollPosition {
             offset_x: vec.x,
             offset_y: vec.y,