Auto merge of #25560 - mrobinson:scrollable-overflow-v1, r=SimonSapin

Add initial scrolling support to layout_2020

This adds very early scrolling support to layout_2020. Only the root element scrolls for now, but the idea is to continue to polish this.

---
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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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1 files changed, 36 insertions, 0 deletions

diff --git a/components/layout_2020/geom.rs b/components/layout_2020/geom.rs
index 166ba7a0494..918cbf47fd2 100644
--- a/components/layout_2020/geom.rs
+++ b/components/layout_2020/geom.rs
@@ -60,6 +60,15 @@ pub(crate) mod flow_relative {
     }
 }
 
+impl<T: Zero> physical::Vec2<T> {
+    pub fn zero() -> Self {
+        Self {
+            x: T::zero(),
+            y: T::zero(),
+        }
+    }
+}
+
 impl<T: fmt::Debug> fmt::Debug for physical::Vec2<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         // Not using f.debug_struct on purpose here, to keep {:?} output somewhat compact
@@ -387,6 +396,33 @@ impl<T> physical::Rect<T> {
     }
 }
 
+impl physical::Rect<Length> {
+    pub fn axis_aligned_bounding_box(&self, other: &Self) -> Self {
+        let top_left = physical::Vec2 {
+            x: self.top_left.x.min(other.top_left.x),
+            y: self.top_left.y.min(other.top_left.y),
+        };
+
+        let bottom_corner_x = (self.top_left.x + self.size.x).max(other.top_left.x + other.size.x);
+        let bottom_corner_y = (self.top_left.y + self.size.y).max(other.top_left.y + other.size.y);
+        let size = physical::Vec2 {
+            x: bottom_corner_x - top_left.x,
+            y: bottom_corner_y - top_left.y,
+        };
+
+        Self { top_left, size }
+    }
+}
+
+impl<T: Zero> physical::Rect<T> {
+    pub fn zero() -> Self {
+        Self {
+            top_left: physical::Vec2::zero(),
+            size: physical::Vec2::zero(),
+        }
+    }
+}
+
 impl From<physical::Rect<Length>> for Rect<CSSPixel> {
     fn from(r: physical::Rect<Length>) -> Self {
         Rect {