Improve ComputedNode accessibility (#16738)

# Objective

- Enable modifying node size after layout.
- Gain access to a node's content_size. `UiSurface` is a private type so
content size can't be looked up.

## Solution

- Make `ComputedNode` fields public.
- Add `content_size` to `ComputedNode`.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_ui/src/layout/mod.rs

@@ -357,6 +357,9 @@ with UI components as a child of an entity without UI components, your UI layout
                 node.inverse_scale_factor = inverse_target_scale_factor;
             }
 
+            let content_size = Vec2::new(layout.content_size.width, layout.content_size.height);
+            node.bypass_change_detection().content_size = content_size;
+
             let taffy_rect_to_border_rect = |rect: taffy::Rect<f32>| BorderRect {
                 left: rect.left,
                 right: rect.right,
@@ -423,7 +426,6 @@ with UI components as a child of an entity without UI components, your UI layout
                 })
                 .unwrap_or_default();
 
-            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,

crates/bevy_ui/src/ui_node.rs

@@ -18,51 +18,62 @@ use smallvec::SmallVec;
 use thiserror::Error;
 
 /// Provides the computed size and layout properties of the node.
+///
+/// Fields in this struct are public but should not be modified under most circumstances.
+/// For example, in a scrollbar you may want to derive the handle's size from the proportion of
+/// scrollable content in-view. You can directly modify `ComputedNode` after layout to set the
+/// handle size without any delays.
 #[derive(Component, Debug, Copy, Clone, PartialEq, Reflect)]
 #[reflect(Component, Default, Debug)]
 pub struct ComputedNode {
     /// The order of the node in the UI layout.
     /// Nodes with a higher stack index are drawn on top of and receive interactions before nodes with lower stack indices.
-    pub(crate) stack_index: u32,
-    /// The size of the node as width and height in physical pixels
     ///
-    /// automatically calculated by [`super::layout::ui_layout_system`]
-    pub(crate) size: Vec2,
+    /// Automatically calculated in [`super::UiSystem::Stack`].
+    pub stack_index: u32,
+    /// The size of the node as width and height in physical pixels.
+    ///
+    /// Automatically calculated by [`super::layout::ui_layout_system`].
+    pub size: Vec2,
+    /// Size of this node's content.
+    ///
+    /// Automatically calculated by [`super::layout::ui_layout_system`].
+    pub content_size: Vec2,
     /// The width of this node's outline.
     /// If this value is `Auto`, negative or `0.` then no outline will be rendered.
     /// Outline updates bypass change detection.
     ///
     /// Automatically calculated by [`super::layout::ui_layout_system`].
-    pub(crate) outline_width: f32,
+    pub outline_width: f32,
     /// The amount of space between the outline and the edge of the node.
     /// Outline updates bypass change detection.
     ///
     /// Automatically calculated by [`super::layout::ui_layout_system`].
-    pub(crate) outline_offset: f32,
+    pub outline_offset: f32,
     /// The unrounded size of the node as width and height in physical pixels.
     ///
     /// Automatically calculated by [`super::layout::ui_layout_system`].
-    pub(crate) unrounded_size: Vec2,
-    /// Resolved border values in physical pixels
+    pub unrounded_size: Vec2,
+    /// Resolved border values in physical pixels.
     /// Border updates bypass change detection.
     ///
     /// Automatically calculated by [`super::layout::ui_layout_system`].
-    pub(crate) border: BorderRect,
+    pub border: BorderRect,
     /// Resolved border radius values in physical pixels.
     /// Border radius updates bypass change detection.
     ///
     /// Automatically calculated by [`super::layout::ui_layout_system`].
-    pub(crate) border_radius: ResolvedBorderRadius,
-    /// Resolved padding values in physical pixels
+    pub border_radius: ResolvedBorderRadius,
+    /// Resolved padding values in physical pixels.
     /// Padding updates bypass change detection.
     ///
     /// Automatically calculated by [`super::layout::ui_layout_system`].
-    pub(crate) padding: BorderRect,
+    pub padding: BorderRect,
     /// Inverse scale factor for this Node.
     /// Multiply physical coordinates by the inverse scale factor to give logical coordinates.
     ///
     /// Automatically calculated by [`super::layout::ui_layout_system`].
-    pub(crate) inverse_scale_factor: f32,
+    pub inverse_scale_factor: f32,
 }
 
 impl ComputedNode {
@@ -74,6 +85,14 @@ impl ComputedNode {
         self.size
     }
 
+    /// The calculated node content size as width and height in physical pixels.
+    ///
+    /// Automatically calculated by [`super::layout::ui_layout_system`].
+    #[inline]
+    pub const fn content_size(&self) -> Vec2 {
+        self.content_size
+    }
+
     /// Check if the node is empty.
     /// A node is considered empty if it has a zero or negative extent along either of its axes.
     #[inline]
@@ -215,6 +234,7 @@ impl ComputedNode {
     pub const DEFAULT: Self = Self {
         stack_index: 0,
         size: Vec2::ZERO,
+        content_size: Vec2::ZERO,
         outline_width: 0.,
         outline_offset: 0.,
         unrounded_size: Vec2::ZERO,