Fix UI accessibility transforms and update ordering (#23859)

# Objective

* Bevy UI's accessibility module doesn't set the transform on
accessiblity nodes, ignoring scaling and rotation.
* UI accessibility nodes should be updated before `bevy_winit` updates
the accesskit adaptors, otherwise there will be a frames delay.


## Solution

Replaced the `calc_bounds` systems with a new system
`sync_bounds_and_transforms`.

Each accesskit `Node` corresponding to an `AccessibleNode` UI entity is
now given object-centered coordinates for its bounding rect (instead of
window coordinates) and a transform.

Accesskit uses local transforms so if an accessible node also has an
accessible parent, its transform has to be recomputed relative to its
parent.

## Testing

I modified the button example so that accesskit integration is enabled
by default and the button is drawn at a 45 degrees angle.

```
cargo run --example button
```

Screen readers should only react when the pointer is directly over the
rotated button if the changes are working.

The example changes should be reverted before merging.

crates/bevy_ui/src/accessibility.rs

@@ -5,18 +5,20 @@ use crate::{
     widget::{ImageNode, TextUiReader},
     ComputedNode, UiSystems,
 };
-use bevy_a11y::AccessibilityNode;
+use bevy_a11y::{AccessibilityNode, AccessibilitySystems};
 use bevy_app::{App, Plugin, PostUpdate};
 use bevy_ecs::{
-    prelude::{DetectChanges, Entity},
-    query::{Changed, Without},
+    change_detection::DetectChanges,
+    hierarchy::ChildOf,
+    prelude::Entity,
+    query::{Changed, With, Without},
     schedule::IntoScheduleConfigs,
     system::{Commands, Query},
     world::Ref,
 };
+use bevy_math::Affine2;
 
-use accesskit::{Node, Rect, Role};
-use bevy_camera::CameraUpdateSystems;
+use accesskit::{Affine, Node, Rect, Role};
 
 fn calc_label(
     text_reader: &mut TextUiReader,
@@ -35,21 +37,43 @@ fn calc_label(
     name.map(String::into_boxed_str)
 }
 
-fn calc_bounds(
-    mut nodes: Query<(
+fn sync_bounds_and_transforms(
+    mut accessible_nodes_query: Query<(
         &mut AccessibilityNode,
         Ref<ComputedNode>,
         Ref<UiGlobalTransform>,
+        Option<&ChildOf>,
     )>,
+    accessible_transform_query: Query<Ref<UiGlobalTransform>, With<AccessibilityNode>>,
 ) {
-    for (mut accessible, node, transform) in &mut nodes {
-        if node.is_changed() || transform.is_changed() {
-            let center = transform.translation;
-            let half_size = 0.5 * node.size;
-            let min = center - half_size;
-            let max = center + half_size;
-            let bounds = Rect::new(min.x as f64, min.y as f64, max.x as f64, max.y as f64);
-            accessible.set_bounds(bounds);
+    for (mut accessible, node, ui_transform, maybe_child_of) in &mut accessible_nodes_query {
+        let maybe_parent_transform = maybe_child_of
+            .and_then(|child_of| accessible_transform_query.get(child_of.parent()).ok());
+
+        if !(node.is_changed()
+            || ui_transform.is_changed()
+            || maybe_parent_transform.is_some_and(|transform| transform.is_changed()))
+        {
+            continue;
+        }
+
+        accessible.set_bounds(Rect::new(
+            -0.5 * node.size.x as f64,
+            -0.5 * node.size.y as f64,
+            0.5 * node.size.x as f64,
+            0.5 * node.size.y as f64,
+        ));
+
+        // If the node has an accessible parent, its transform in the accessibility tree must be relative to the parent.
+        let transform = maybe_parent_transform
+            .and_then(|transform| transform.try_inverse())
+            .unwrap_or_default()
+            * ui_transform.affine();
+
+        if transform.is_finite() && transform != Affine2::IDENTITY {
+            accessible.set_transform(Affine::new(transform.to_cols_array().map(f64::from)));
+        } else {
+            accessible.clear_transform();
         }
     }
 }
@@ -149,16 +173,18 @@ impl Plugin for AccessibilityPlugin {
         app.add_systems(
             PostUpdate,
             (
-                calc_bounds
-                    .after(bevy_transform::TransformSystems::Propagate)
-                    .after(CameraUpdateSystems)
-                    // the listed systems do not affect calculated size
-                    .ambiguous_with(crate::ui_stack_system)
-                    .before(UiSystems::PostLayout),
                 button_changed,
                 image_changed,
                 label_changed,
-            ),
+                sync_bounds_and_transforms
+                    .after(button_changed)
+                    .after(image_changed)
+                    .after(label_changed)
+                    // the listed systems do not affect calculated size
+                    .ambiguous_with(crate::ui_stack_system),
+            )
+                .in_set(UiSystems::PostLayout)
+                .before(AccessibilitySystems::Update),
         );
     }
 }

examples/ui/widgets/button.rs

@@ -8,6 +8,12 @@ fn main() {
         .add_plugins(DefaultPlugins)
         // `InputFocus` must be set for accessibility to recognize the button.
         .init_resource::<InputFocus>()
+        .add_systems(
+            Startup,
+            |requested: Res<bevy::a11y::AccessibilityRequested>| {
+                requested.set(true);
+            },
+        )
         .add_systems(Startup, setup)
         .add_systems(Update, button_system)
         .run();
@@ -92,6 +98,11 @@ fn button(asset_server: &AssetServer) -> impl Bundle {
                 border_radius: BorderRadius::MAX,
                 ..default()
             },
+            UiTransform {
+                translation: default(),
+                scale: 2. * Vec2::ONE,
+                rotation: Rot2::degrees(45.)
+            },
             BorderColor::all(Color::WHITE),
             BackgroundColor(Color::BLACK),
             children![(