Call `ensure_no_corner_overlap()` to legalize border radii in

`ClipItem::new_box_shadow()`.

If no blur is present, `DisplayListFlattener::add_box_shadow()` calls
`ClipItem::new_rounded_rect()` in its fast path, which calls
`ensure_no_corner_overlap()`. However, in the slow path, which is used when
blur is present, `ClipItem::new_box_shadow()` is called instead. Prior to this
patch, `ClipItem::new_box_shadow()` neglected to call
`ensure_no_corner_overlap()`. This is why the bug manifested itself only when
blur was present.

Closes #2581.

webrender/src/clip.rs

@@ -765,11 +765,14 @@ impl ClipItem {
 
     pub fn new_box_shadow(
         shadow_rect: LayoutRect,
-        shadow_radius: BorderRadius,
+        mut shadow_radius: BorderRadius,
         prim_shadow_rect: LayoutRect,
         blur_radius: f32,
         clip_mode: BoxShadowClipMode,
     ) -> Self {
+        // Make sure corners don't overlap.
+        ensure_no_corner_overlap(&mut shadow_radius, &shadow_rect);
+
         // Get the fractional offsets required to match the
         // source rect with a minimal rect.
         let fract_offset = LayoutPoint::new(

wrench/reftests/boxshadow/box-shadow-blurred-overlapping-radii-ref.yaml

@@ -0,0 +1,20 @@
+
+---
+root:
+  items:
+    - type: stacking-context
+      bounds: [0, 0, 1000, 1000]
+      items:
+        - type: box-shadow
+          bounds: [ 0, 1, 265, 265 ]
+          color: red
+          clip-mode: outset
+          offset: 350 -1
+          blur-radius: 1
+          spread-radius: -80
+          border-radius:
+            top-left: [185, 185]
+            top-right: [0, 0]
+            bottom-left: [0, 0]
+            bottom-right: [185, 185]
+

wrench/reftests/boxshadow/box-shadow-blurred-overlapping-radii.yaml

@@ -0,0 +1,19 @@
+
+---
+root:
+  items:
+    - type: stacking-context
+      bounds: [0, 0, 1000, 1000]
+      items:
+        - type: box-shadow
+          bounds: [ 0, 1, 265, 265 ]
+          color: red
+          clip-mode: outset
+          offset: 350 -1
+          blur-radius: 1
+          spread-radius: -80
+          border-radius:
+            top-left: [250, 250]
+            top-right: [0, 0]
+            bottom-left: [0, 0]
+            bottom-right: [250, 250]

wrench/reftests/boxshadow/reftest.list

@@ -24,8 +24,8 @@ platform(linux,mac) fuzzy(1,4) == inset-downscale.yaml inset-downscale.png
 platform(linux,mac) == box-shadow-cache.yaml box-shadow-cache.png
 platform(linux,mac) fuzzy(1,685) == overlap1.yaml overlap1.png
 == overlap2.yaml overlap2.png
-platform(linux,mac) == no-stretch.yaml no-stretch.png
+platform(linux,mac) fuzzy(1,48) == no-stretch.yaml no-stretch.png
 platform(linux,mac) == box-shadow-stretch-mode-x.yaml box-shadow-stretch-mode-x.png
 platform(linux,mac) == box-shadow-stretch-mode-y.yaml box-shadow-stretch-mode-y.png
 platform(linux,mac) == inset-mask-region.yaml inset-mask-region.png
-
+== box-shadow-blurred-overlapping-radii.yaml box-shadow-blurred-overlapping-radii-ref.yaml