Only paint the background onto the scrolling contents layer if we have a scrolling contents layer.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMappingTest.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/layout/compositing/CompositedLayerMapping.h"
 
 #include "core/frame/FrameView.h"
 #include "core/layout/LayoutBoxModelObject.h"
 #include "core/layout/LayoutTestHelper.h"
 #include "core/layout/api/LayoutViewItem.h"
@@ skipping 629 matching lines @@
     Element* negativeCompositedChild = document().getElementById("negative-composited-child");
     negativeCompositedChild->parentNode()->removeChild(negativeCompositedChild);
     document().view()->updateAllLifecyclePhases();
 
     mapping = toLayoutBlock(getLayoutObjectByElementId("container"))->layer()->compositedLayerMapping();
     ASSERT_TRUE(mapping->scrollingContentsLayer());
     EXPECT_EQ(static_cast<GraphicsLayerPaintingPhase>(GraphicsLayerPaintOverflowContents | GraphicsLayerPaintCompositedScroll | GraphicsLayerPaintForeground), mapping->scrollingContentsLayer()->paintingPhase());
     EXPECT_FALSE(mapping->foregroundLayer());
 }
 
+TEST_F(CompositedLayerMappingTest, BackgroundPaintedIntoGraphicsLayerIfNotCompositedScrolling)
+{
+    document().frame()->settings()->setPreferCompositingToLCDTextEnabled(true);
+    setBodyInnerHTML(
+        "<div id='container' style='overflow: scroll; width: 300px; height: 300px; border-radius: 5px; background: white; will-change: transform;'>"
+        "    <div style='background-color: blue; width: 2000px; height: 2000px;'></div>"
+        "</div>");
+
+    PaintLayer* layer = toLayoutBlock(getLayoutObjectByElementId("container"))->layer();
+    EXPECT_TRUE(layer->canPaintBackgroundOntoScrollingContentsLayer());
+
+    // We currently don't use composited scrolling when the container has a border-radius
+    // so even though we can paint the background onto the scrolling contents layer we
+    // don't have a scrolling contents layer to paint into in this case.
+    CompositedLayerMapping* mapping = layer->compositedLayerMapping();
+    EXPECT_FALSE(mapping->hasScrollingLayer());
+    EXPECT_FALSE(mapping->backgroundPaintsOntoScrollingContentsLayer());
+}
+
 // Make sure that clipping layers are removed or their masking bit turned off
 // when they're an ancestor of the root scroller element.
 TEST_F(CompositedLayerMappingTest, RootScrollerAncestorsNotClipped)
 {
     NonThrowableExceptionState nonThrow;
 
     TopDocumentRootScrollerController& rootScrollerController =
         document().frameHost()->globalRootScrollerController();
 
     setBodyInnerHTML(
@@ skipping 109 matching lines @@
         document().setRootScroller(nullptr, nonThrow);
         document().view()->updateAllLifecyclePhases();
         ASSERT_EQ(document().documentElement(), rootScrollerController.globalRootScroller());
 
         EXPECT_TRUE(mapping3->clippingLayer());
         EXPECT_TRUE(mapping3->clippingLayer()->platformLayer()->masksToBounds());
     }
 }
 
 } // namespace blink