ComputedNode box model helper functions (#21903)

# Objective

Add helper functions to `ComputedNode` that return the bounds of the
node's border, padding, and content areas.

## Solution

Add helper functions `border_box`, `padding_box`, and `content_box` to
`ComputedNode`.

#

Included a release note. These changes are very trivial and don't really
need one, but I've got quite a few more less trivial changes to
`ComputedNode` that I want to add later.

## Testing

This PR also includes a couple of trivial tests.

Author: ickshonpe

crates/bevy_ui/src/ui_node.rs

@@ -302,6 +302,38 @@ impl ComputedNode {
         clip_rect
     }
 
+    /// Returns the node's border-box in object-centered physical coordinates.
+    /// This is the full rectangle enclosing the node.
+    #[inline]
+    pub fn border_box(&self) -> Rect {
+        Rect::from_center_size(Vec2::ZERO, self.size)
+    }
+
+    /// Returns the node's padding-box in object-centered physical coordinates.
+    /// This is the region inside the border containing the node's padding and content areas.
+    #[inline]
+    pub fn padding_box(&self) -> Rect {
+        let mut out = self.border_box();
+        out.min.x += self.border.left;
+        out.max.x -= self.border.right;
+        out.min.y += self.border.top;
+        out.max.y -= self.border.bottom;
+        out
+    }
+
+    /// Returns the node's content-box in object-centered physical coordinates.
+    /// This is the innermost region of the node, where its content is placed.
+    #[inline]
+    pub fn content_box(&self) -> Rect {
+        let mut out = self.border_box();
+        let content_inset = self.content_inset();
+        out.min.x += content_inset.left;
+        out.max.x -= content_inset.right;
+        out.min.y += content_inset.top;
+        out.max.y -= content_inset.bottom;
+        out
+    }
+
     const fn compute_thumb(
         gutter_min: f32,
         content_length: f32,
@@ -2987,11 +3019,10 @@ impl ComputedUiRenderTargetInfo {
 
 #[cfg(test)]
 mod tests {
-    use bevy_math::Rect;
-    use bevy_math::Vec2;
-
     use crate::ComputedNode;
     use crate::GridPlacement;
+    use bevy_math::{Rect, Vec2};
+    use bevy_sprite::BorderRect;
 
     #[test]
     fn invalid_grid_placement_values() {
@@ -3116,4 +3147,52 @@ mod tests {
         );
         assert_eq!(thumb, [0., 50.]);
     }
+
+    #[test]
+    fn border_box_is_centered_rect_of_node_size() {
+        let node = ComputedNode {
+            size: Vec2::new(100.0, 50.0),
+            ..Default::default()
+        };
+        let border_box = node.border_box();
+
+        assert_eq!(border_box.min, Vec2::new(-50.0, -25.0));
+        assert_eq!(border_box.max, Vec2::new(50.0, 25.0));
+    }
+
+    #[test]
+    fn padding_box_subtracts_border_thickness() {
+        let node = ComputedNode {
+            size: Vec2::new(100.0, 60.0),
+            border: BorderRect {
+                left: 5.0,
+                right: 7.0,
+                top: 3.0,
+                bottom: 9.0,
+            },
+            ..Default::default()
+        };
+        let padding_box = node.padding_box();
+
+        assert_eq!(padding_box.min, Vec2::new(-50.0 + 5.0, -30.0 + 3.0));
+        assert_eq!(padding_box.max, Vec2::new(50.0 - 7.0, 30.0 - 9.0));
+    }
+
+    #[test]
+    fn content_box_uses_content_inset() {
+        let node = ComputedNode {
+            size: Vec2::new(80.0, 40.0),
+            padding: BorderRect {
+                left: 4.0,
+                right: 6.0,
+                top: 2.0,
+                bottom: 8.0,
+            },
+            ..Default::default()
+        };
+        let content_box = node.content_box();
+
+        assert_eq!(content_box.min, Vec2::new(-40.0 + 4.0, -20.0 + 2.0));
+        assert_eq!(content_box.max, Vec2::new(40.0 - 6.0, 20.0 - 8.0));
+    }
 }

release-content/release-notes/computed_node_helper_functions.md

@@ -0,0 +1,7 @@
+---
+title: "`ComputedNode` helper functions"
+authors: ["@ickshonpe"]
+pull_requests: [21903]
+---
+
+Helper functions `border_box`, `padding_box`, and `content_box` that return a node’s object-centered border, padding, and content boxes have been added to `ComputedNode`.