cc: Enable composited border-radius scrolling.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 534 matching lines @@
 void CompositedLayerMapping::UpdateCompositingReasons() {
   // All other layers owned by this mapping will have the same compositing
   // reason for their lifetime, so they are initialized only when created.
   graphics_layer_->SetCompositingReasons(owning_layer_.GetCompositingReasons());
   graphics_layer_->SetSquashingDisallowedReasons(
       owning_layer_.GetSquashingDisallowedReasons());
 }
 
 bool CompositedLayerMapping::AncestorRoundedCornersWontClip(
     const LayoutBoxModelObject& child,
     const LayoutBoxModelObject& clipping_ancestor) {

[trchen, 2017/05/22 23:00:06]

This has to be changed as well. If there is anything that can change relative
position between child and clipping_ancestor, overlap must be assumed (thus clip
cannot be omitted). For example: http://jsbin.com/rexelunugo/

[sunxd, 2017/06/15 15:59:10]

There was a bug about nested border-radius clipping, schenney@ recently landed
the fix. I think the position issue is fixed?

[Stephen Chennney, 2017/06/15 17:44:24]

Yes, the nested border radius mask code will look at every layer up to the
composited ancestor now, not just the clipping ancestor.

[sunxd, 2017/06/15 19:59:20]

Done.

   LayoutRect local_visual_rect = composited_bounds_;
   child.MapToVisualRectInAncestorSpace(&clipping_ancestor, local_visual_rect);
   FloatRoundedRect rounded_clip_rect =
       clipping_ancestor.Style()->GetRoundedInnerBorderFor(
           clipping_ancestor.LocalVisualRect());
   FloatRect inner_clip_rect = rounded_clip_rect.RadiusCenterRect();
   // The first condition catches cases where the child is certainly inside
   // the rounded corner portion of the border, and cannot be clipped by
   // the rounded portion. The second catches cases where the child is
   // entirely outside the rectangular border (ignoring rounded corners) so
@@ skipping 18 matching lines @@
   const PaintLayer* compositing_ancestor =
       owning_layer_.EnclosingLayerWithCompositedLayerMapping(kExcludeSelf);
   if (!compositing_ancestor)
     return;
 
   const LayoutBoxModelObject* clipping_container =
       owning_layer_.ClippingContainer();
   if (!clipping_container)
     return;
 
-  if (clipping_container->EnclosingLayer() == scroll_parent)
-    return;
-
   if (compositing_ancestor->GetLayoutObject().IsDescendantOf(
           clipping_container))
     return;
 
   // We ignore overflow clip here; we want composited overflow content to
   // behave as if it lives in an unclipped universe so it can prepaint, etc.
   // This means that we need to check if we are actually clipped before
   // setting up m_ancestorClippingLayer otherwise
   // updateAncestorClippingLayerGeometry will fail as the clip rect will be
   // infinite.
@@ skipping 10 matching lines @@
   owning_layer_is_clipped = parent_clip_rect != LayoutRect::InfiniteIntRect();
 
   // TODO(schenney): CSS clips are not applied to composited children, and
   // should be via mask or by compositing the parent too.
   // https://bugs.chromium.org/p/chromium/issues/detail?id=615870
   DCHECK(clipping_container->Style());
   owning_layer_is_masked =
       owning_layer_is_clipped &&
       clipping_container->Style()->HasBorderRadius() &&
       !AncestorRoundedCornersWontClip(GetLayoutObject(), *clipping_container);
+
+  if (clipping_container->EnclosingLayer() == scroll_parent)
+    owning_layer_is_clipped = owning_layer_is_masked;

[trchen, 2017/05/22 23:00:06]

Could you document what does this line do? Actually I don't understand this
either before your CL or after your CL.

[sunxd, 2017/06/15 15:59:10]

Sure. It was added due to this bug: crbug.com/450756.
I think it's trying to avoid applying scroll parent as the ancestor clipping
layer to composited scrollers.

[Stephen Chennney, 2017/06/15 17:44:24]

Will we not now have the same problem as crbug.com/450756 in cases where the
mask is required? Could you make a test based off
compositing/overflow/avoid-ancestor-clip-for-scroll-children.html that has a
border radius to invoke the mask?

[sunxd, 2017/06/15 19:59:20]

Oh sorry I think this change should be reverted as the new fix is going to take
care of the mask problem of scroll_parents.

 }
 
 const PaintLayer* CompositedLayerMapping::ScrollParent() {
   const PaintLayer* scroll_parent = owning_layer_.ScrollParent();
   if (scroll_parent && !scroll_parent->NeedsCompositedScrolling())
     return nullptr;
   return scroll_parent;
 }
 
 bool CompositedLayerMapping::UpdateGraphicsLayerConfiguration() {
@@ skipping 3025 matching lines @@
   } else if (graphics_layer == decoration_outline_layer_.get()) {
     name = "Decoration Layer";
   } else {
     NOTREACHED();
   }
 
   return name;
 }
 
 }  // namespace blink