--- a/gfx/wr/webrender_api/src/display_item.rs
+++ b/gfx/wr/webrender_api/src/display_item.rs
@@ -1297,22 +1297,24 @@ impl BorderRadius {
             self.bottom_right == uniform_radius
         {
             Some(uniform_radius)
         } else {
             None
         }
     }
 
+    /// Return whether, in each corner, the radius in *either* direction is zero.
+    /// This means that none of the corners are rounded.
     pub fn is_zero(&self) -> bool {
-        if let Some(radius) = self.is_uniform() {
-            radius == 0.0
-        } else {
-            false
-        }
+        let corner_is_zero = |corner: &LayoutSize| corner.width == 0.0 || corner.height == 0.0;
+        corner_is_zero(&self.top_left) &&
+        corner_is_zero(&self.top_right) &&
+        corner_is_zero(&self.bottom_right) &&
+        corner_is_zero(&self.bottom_left)
     }
 }
 
 impl ComplexClipRegion {
     /// Create a new complex clip region.
     pub fn new(
         rect: LayoutRect,
         radii: BorderRadius,