Don't calculate inner rectangles for non-axis-aligned clips

webrender/src/clip.rs

@@ -10,7 +10,7 @@ use freelist::{FreeList, FreeListHandle, WeakFreeListHandle};
 use gpu_cache::{GpuCache, GpuCacheHandle, ToGpuBlocks};
 use prim_store::{ClipData, ImageMaskData};
 use resource_cache::ResourceCache;
-use util::{MaxRect, calculate_screen_bounding_rect, extract_inner_rect_safe};
+use util::{MatrixHelpers, MaxRect, calculate_screen_bounding_rect, extract_inner_rect_safe};
 
 pub type ClipStore = FreeList<ClipSources>;
 pub type ClipSourcesHandle = FreeListHandle<ClipSources>;
@@ -251,8 +251,20 @@ impl ClipSources {
         transform: &LayerToWorldTransform,
         device_pixel_ratio: f32,
     ) -> (DeviceIntRect, Option<DeviceIntRect>) {
-        let screen_inner_rect =
-            calculate_screen_bounding_rect(transform, &self.local_inner_rect, device_pixel_ratio);
+        // If this translation isn't axis aligned or has a perspective component, don't try to
+        // calculate the inner rectangle. The rectangle that we produce would include potentially
+        // clipped screen area.
+        // TODO(mrobinson): We should eventually try to calculate an inner region or some inner
+        // rectangle so that we can do screen inner rectangle optimizations for these kind of
+        // cilps.
+        let can_calculate_inner_rect =
+            transform.preserves_2d_axis_alignment() && !transform.has_perspective_component();
+        let screen_inner_rect = if can_calculate_inner_rect {
+            calculate_screen_bounding_rect(transform, &self.local_inner_rect, device_pixel_ratio)
+        } else {
+            DeviceIntRect::zero()
+        };
+
         let screen_outer_rect = self.local_outer_rect.map(|outer_rect|
             calculate_screen_bounding_rect(transform, &outer_rect, device_pixel_ratio)
         );

wrench/reftests/clip/clip-45-degree-rotation.yaml

@@ -0,0 +1,35 @@
+# Test that transformed content is clipped properly by clips with a different transform.
+---
+root:
+  items:
+    -
+      bounds: [0, 0, 0, 0]
+      "clip-rect": [0, 0, 0, 0]
+      "clip-and-scroll": 0
+      type: "stacking-context"
+      transform: rotate(45) translate(200, 0)
+      items:
+        -
+          bounds: [0, 0, 300, 300]
+          "clip-rect": [0, 0, 300, 300]
+          "clip-and-scroll": 0
+          type: rect
+          color: red
+        -
+          bounds: [0, 0, 300, 300]
+          "clip-rect": [0, 0, 300, 300]
+          "clip-and-scroll": 0
+          type: clip
+          id: 5
+        -
+          bounds: [0, 0, 0, 0]
+          "clip-rect": [0, 0, 0, 0]
+          "clip-and-scroll": 5
+          type: "stacking-context"
+          transform: rotate(-45) translate(-300, 0)
+          items:
+            -
+              bounds: [0, 0, 1598, 1200]
+              "clip-rect": [0, 0, 1598, 1200]
+              type: rect
+              color: green

wrench/reftests/clip/reftest.list

@@ -1,3 +1,4 @@
 == clip-mode.yaml clip-mode.png
 == clip-ellipse.yaml clip-ellipse.png
+== clip-45-degree-rotation.yaml clip-45-degree-rotation-ref.png