Fix clipping of squashed sibling layers with border radius masks

third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintLayerPainter.h"
 
 #include "core/frame/LocalFrame.h"
 #include "core/layout/LayoutView.h"
 #include "core/paint/ClipPathClipper.h"
 #include "core/paint/FilterPainter.h"
@@ skipping 943 matching lines @@
   Optional<LayerClipRecorder> clip_recorder;
   if (clip_state != kHasClipped && painting_info.clip_to_dirty_rect &&
       NeedsToClip(painting_info, clip_rect)) {
     DisplayItem::Type clip_type =
         DisplayItem::PaintPhaseToClipLayerFragmentType(phase);
     LayerClipRecorder::BorderRadiusClippingRule clipping_rule;
     switch (phase) {
       case kPaintPhaseSelfBlockBackgroundOnly:  // Background painting will
                                                 // handle clipping to self.
       case kPaintPhaseSelfOutlineOnly:
-      case kPaintPhaseMask:  // Mask painting will handle clipping to self.
+      case kPaintPhaseMask:
+      case kPaintPhaseClippingMask:

[chrishtr, 2017/04/17 17:25:00]

Your fix is generally correct, just a small error. kPaintPhaseClippingMask can
be true
for more than just ancestor mask clips. If you use this code instead, it should
work:

case kPaintPhaseClippingMask:
        if (paint_flags & kPaintLayerPaintingAncestorClippingMaskPhase) {
          clipping_rule = LayerClipRecorder::kDoNotIncludeSelfForBorderRadius;
          break;
        }

+        // Mask painting will handle clipping to self.
         clipping_rule = LayerClipRecorder::kDoNotIncludeSelfForBorderRadius;
         break;
       default:
         clipping_rule = LayerClipRecorder::kIncludeSelfForBorderRadius;
         break;
     }
 
     // TODO(schenney): Nested border-radius clips are not applied to composited
     // children, probably due to an incorrect clipRoot.
     // https://bugs.chromium.org/p/chromium/issues/detail?id=672561
@@ skipping 243 matching lines @@
           context, layout_object, kPaintPhaseClippingMask))
     return;
 
   IntRect snapped_clip_rect = PixelSnappedIntRect(clip_rect.Rect());
   LayoutObjectDrawingRecorder drawing_recorder(
       context, layout_object, kPaintPhaseClippingMask, snapped_clip_rect);
   context.FillRect(snapped_clip_rect, Color::kBlack);
 }
 
 }  // namespace blink