[blink] Skip ancestor clip in PaintLayerPainter for squashed layers.

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 740 matching lines @@
     if (layer_fragments.size() > 1)
       result = kMayBeClippedByPaintDirtyRect;
   }
 
   Optional<DisplayItemCacheSkipper> cache_skipper;
   if (layer_fragments.size() > 1)
     cache_skipper.emplace(context);
 
   ClipRect ancestor_background_clip_rect;
   if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled()) {
-    if (parent_layer) {
+    if (painting_info.root_layer == &paint_layer_) {
+      // This workaround a bug in squash layer painting.

[chrishtr, 2017/04/23 17:13:44]

Nit: "This works around a bug in squashed-layer painting"

+      // Squashed layers paint into a backing in its compositing container's
+      // space, but painting_info.root_layer points to the squashed layer
+      // itself, thus PaintLayerClipper would return a clip rect in the
+      // squashed layer's local space, instead of the backing's space.
+      // Fortunately, CompositedLayerMapping::DoPaintTask already applied
+      // appropriate ancestor clip for us, we can simply skip it.
+      DCHECK_EQ(paint_layer_.GetCompositingState(), kPaintsIntoGroupedBacking);
+      ancestor_background_clip_rect.SetRect(FloatClipRect());
+    } else if (parent_layer) {
       // Calculate the clip rectangle that the ancestors establish.
       ClipRectsContext clip_rects_context(
           painting_info.root_layer,
           (paint_flags & kPaintLayerUncachedClipRects) ? kUncachedClipRects
                                                        : kPaintingClipRects,
           kIgnorePlatformOverlayScrollbarSize);
       if (ShouldRespectOverflowClip(paint_flags,
                                     paint_layer_.GetLayoutObject()) ==
           kIgnoreOverflowClip)
         clip_rects_context.SetIgnoreOverflowClip();
@@ skipping 463 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