Clean up UiSystem system sets (#14228)

# Objective

- All UI systems should be in system sets that are easy to order around
in user code.

## Solution

- Add `UiSystem::Prepare` and `UiSystem::PostLayout` system sets to
capture floater systems.
- Adjust how UI systems are scheduled to align with the new sets.

This is *mostly* a pure refactor without any behavior/scheduling
changes. See migration guide.

## Testing

- Not tested, correctness by inspection.

---

## Migration Guide

`UiSystem` system set adjustments.
- The `UiSystem::Outline` system set is now strictly ordered after
`UiSystem::Layout`, rather than overlapping it.

crates/bevy_ui/src/lib.rs

@@ -70,13 +70,27 @@ pub struct UiPlugin;
 /// The label enum labeling the types of systems in the Bevy UI
 #[derive(Debug, Hash, PartialEq, Eq, Clone, SystemSet)]
 pub enum UiSystem {
-    /// After this label, the ui layout state has been updated
-    Layout,
-    /// After this label, input interactions with UI entities have been updated for this frame
+    /// After this label, input interactions with UI entities have been updated for this frame.
+    ///
+    /// Runs in [`PreUpdate`].
     Focus,
-    /// After this label, the [`UiStack`] resource has been updated
+    /// All UI systems in [`PostUpdate`] will run in or after this label.
+    Prepare,
+    /// After this label, the ui layout state has been updated.
+    ///
+    /// Runs in [`PostUpdate`].
+    Layout,
+    /// UI systems ordered after [`UiSystem::Layout`].
+    ///
+    /// Runs in [`PostUpdate`].
+    PostLayout,
+    /// After this label, the [`UiStack`] resource has been updated.
+    ///
+    /// Runs in [`PostUpdate`].
     Stack,
-    /// After this label, node outline widths have been updated
+    /// After this label, node outline widths have been updated.
+    ///
+    /// Runs in [`PostUpdate`].
     Outlines,
 }
 
@@ -129,6 +143,15 @@ impl Plugin for UiPlugin {
             .register_type::<widget::Label>()
             .register_type::<ZIndex>()
             .register_type::<Outline>()
+            .configure_sets(
+                PostUpdate,
+                (
+                    UiSystem::Prepare.before(UiSystem::Stack),
+                    UiSystem::Layout,
+                    (UiSystem::PostLayout, UiSystem::Outlines),
+                )
+                    .chain(),
+            )
             .add_systems(
                 PreUpdate,
                 ui_focus_system.in_set(UiSystem::Focus).after(InputSystem),
@@ -138,16 +161,14 @@ impl Plugin for UiPlugin {
             PostUpdate,
             (
                 check_visibility::<WithNode>.in_set(VisibilitySystems::CheckVisibility),
-                update_target_camera_system.before(UiSystem::Layout),
-                apply_deferred
-                    .after(update_target_camera_system)
-                    .before(UiSystem::Layout),
+                (update_target_camera_system, apply_deferred)
+                    .chain()
+                    .in_set(UiSystem::Prepare),
                 ui_layout_system
                     .in_set(UiSystem::Layout)
                     .before(TransformSystem::TransformPropagate),
                 resolve_outlines_system
                     .in_set(UiSystem::Outlines)
-                    .after(UiSystem::Layout)
                     // clipping doesn't care about outlines
                     .ambiguous_with(update_clipping_system)
                     .in_set(AmbiguousWithTextSystem),
@@ -164,14 +185,14 @@ impl Plugin for UiPlugin {
                 // its own UiImage, and `widget::text_system` & `bevy_text::update_text2d_layout`
                 // will never modify a pre-existing `Image` asset.
                 widget::update_image_content_size_system
-                    .before(UiSystem::Layout)
+                    .in_set(UiSystem::Prepare)
                     .in_set(AmbiguousWithTextSystem)
                     .in_set(AmbiguousWithUpdateText2DLayout),
                 (
                     texture_slice::compute_slices_on_asset_event,
                     texture_slice::compute_slices_on_image_change,
                 )
-                    .after(UiSystem::Layout),
+                    .in_set(UiSystem::PostLayout),
             ),
         );
 
@@ -203,7 +224,7 @@ fn build_text_interop(app: &mut App) {
         PostUpdate,
         (
             widget::measure_text_system
-                .before(UiSystem::Layout)
+                .in_set(UiSystem::Prepare)
                 // Potential conflict: `Assets<Image>`
                 // In practice, they run independently since `bevy_render::camera_update_system`
                 // will only ever observe its own render target, and `widget::measure_text_system`
@@ -217,7 +238,7 @@ fn build_text_interop(app: &mut App) {
                 // FIXME: Add an archetype invariant for this https://github.com/bevyengine/bevy/issues/1481.
                 .ambiguous_with(widget::update_image_content_size_system),
             widget::text_system
-                .after(UiSystem::Layout)
+                .in_set(UiSystem::PostLayout)
                 .after(bevy_text::remove_dropped_font_atlas_sets)
                 // Text2d and bevy_ui text are entirely on separate entities
                 .ambiguous_with(bevy_text::update_text2d_layout),