If the transform contains perspective component, the clip rect don't affect by this transform.

webrender/src/tiling.rs

@@ -26,7 +26,8 @@ use renderer::ImageBufferKind;
 use resource_cache::{GlyphFetchResult, ResourceCache};
 use std::{cmp, usize, f32, i32};
 use texture_allocator::GuillotineAllocator;
-use util::{TransformedRect, TransformedRectKind};
+use util::{MatrixHelpers, TransformedRect, TransformedRectKind};
+use euclid::rect;
 
 // Special sentinel value recognized by the shader. It is considered to be
 // a dummy task that doesn't mask out anything.
@@ -1686,7 +1687,13 @@ impl PackedLayer {
         screen_rect: &DeviceIntRect,
         device_pixel_ratio: f32,
     ) -> Option<(TransformedRectKind, DeviceIntRect)> {
-        self.local_clip_rect = *local_rect;
+        self.local_clip_rect = if self.transform.has_perspective_component() {
+            // Given a very large rect which means any rect would be inside this rect.
+            // That is, nothing would be clipped.
+            rect(f32::MIN / 2.0, f32::MIN / 2.0, f32::MAX, f32::MAX)
+        } else {
+            *local_rect
+        };
         let xf_rect = TransformedRect::new(local_rect, &self.transform, device_pixel_ratio);
         xf_rect
             .bounding_rect

webrender/src/util.rs

@@ -17,6 +17,7 @@ pub trait MatrixHelpers<Src, Dst> {
     fn transform_rect(&self, rect: &TypedRect<f32, Src>) -> TypedRect<f32, Dst>;
     fn is_identity(&self) -> bool;
     fn preserves_2d_axis_alignment(&self) -> bool;
+    fn has_perspective_component(&self) -> bool;
     fn inverse_project(&self, target: &TypedPoint2D<f32, Dst>) -> Option<TypedPoint2D<f32, Src>>;
     fn inverse_rect_footprint(&self, rect: &TypedRect<f32, Dst>) -> TypedRect<f32, Src>;
 }
@@ -66,6 +67,10 @@ impl<Src, Dst> MatrixHelpers<Src, Dst> for TypedTransform3D<f32, Src, Dst> {
         col0 < 2 && col1 < 2 && row0 < 2 && row1 < 2
     }
 
+    fn has_perspective_component(&self) -> bool {
+         self.m14 != 0.0 || self.m24 != 0.0 || self.m34 != 0.0 || self.m44 != 1.0
+    }
+
     fn inverse_project(&self, target: &TypedPoint2D<f32, Dst>) -> Option<TypedPoint2D<f32, Src>> {
         let m: TypedTransform2D<f32, Src, Dst>;
         m = TypedTransform2D::column_major(

wrench/reftests/split/perspective-clipping-ref.yaml

@@ -0,0 +1,9 @@
+---
+root:
+  items:
+    - type: stacking-context
+      bounds: 0 0 1024 768
+      items:
+        - type: rect
+          bounds: 0 0 1024 768
+          color: green

wrench/reftests/split/perspective-clipping.yaml

@@ -0,0 +1,22 @@
+---
+root:
+  items:
+    - type: stacking-context
+      bounds: 0 0 1024 768
+      items:
+        - type: rect
+          bounds: 0 0 1024 768
+          color: red
+        - type: stacking-context
+          bounds: 0 0 1024 768
+          transform-style: preserve-3d
+          perspective: 300
+          items:
+            - type: stacking-context
+              bounds: 0 0 1024 768
+              # translate-Z(-300) scale(2)
+              transform: 2 0 0 0 0 2 0 0 0 0 2 0 0 0 -300 1
+              items:
+                - type: rect
+                  bounds: 0 0 1024 768
+                  color: green

wrench/reftests/split/reftest.list

@@ -2,4 +2,5 @@
 == order.yaml order-ref.yaml
 == nested.yaml nested-ref.yaml
 == nested-preserve3d-crash.yaml nested-preserve3d-crash.yaml
+== perspective-clipping.yaml perspective-clipping-ref.yaml
 #== cross.yaml cross-ref.yaml #TODO: investigate sub-pixel differences