Don't automatically paint clipped backgrounds into scrolling contents layer.

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

 // Copyright 2016 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/PaintLayerScrollableAreaTest.h"
 
 #include "platform/graphics/GraphicsLayer.h"
 
 namespace blink {
 
@@ skipping 17 matching lines @@
         "<div id='scroller7' class='scroller' style='background: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUg) local, white content-box; padding: 10px;'><div class='spacer'></div></div>"
         "<div id='scroller8' class='scroller' style='background: white border-box;'><div class='spacer'></div></div>"
         "<div id='scroller9' class='scroller' style='background: white border-box; border: 10px solid black;'><div class='spacer'></div></div>"
         "<div id='scroller10' class='scroller' style='background: white border-box; border: 10px solid rgba(0, 0, 0, 0.5);'><div class='spacer'></div></div>"
         "<div id='scroller11' class='scroller' style='background: white content-box;'><div class='spacer'></div></div>"
         "<div id='scroller12' class='scroller' style='background: white content-box; padding: 10px;'><div class='spacer'></div></div>"
         "<div id='scroller13' class='scroller' style='background: white border-box;'><div class='spacer'></div></div>"
         "<div id='scroller14' class='scroller' style='background: white; border: 1px solid black; outline: 1px solid blue; outline-offset: -1px;'><div class='spacer'></div></div>"
         "<div id='scroller15' class='scroller' style='background: white; border: 1px solid black; outline: 1px solid blue; outline-offset: -2px;'><div class='spacer'></div></div>"
         "<div id='scroller16' class='scroller' style='background: white; border 1px solid black; border-radius: 2px;'><div class='spacer'></div></div>"
+        "<div id='scroller17' class='scroller' style='background: white; clip: rect(0px,10px,10px,0px);'><div class='spacer'></div></div>"
         );
 
     // #scroller1 cannot paint background into scrolling contents layer because it has a negative z-index child.
     EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller1"));
 
     // #scroller2 cannot paint background into scrolling contents layer because it has a content-box clip without local attachment.
     EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller2"));
 
     // #scroller3 can paint background into scrolling contents layer.
     EXPECT_TRUE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller3"));
@@ skipping 40 matching lines @@
     // padding box.
     EXPECT_TRUE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller14"));
 
     // #scroller15 cannot paint background into scrolling contents layer because the outline is drawn inside
     // the padding box.
     EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller15"));
 
     // #scroller16 cannot paint background into scrolling contents layer because it has a
     // border radius.
     EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller16"));
+
+    // #scroller17 cannot paint background into scrolling contents layer because the scroller has a clip which
+    // would not be respected by the scrolling contents layer.
+    EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller17"));
 }
 
 TEST_F(PaintLayerScrollableAreaTest, OpaqueLayersPromoted)
 {
     RuntimeEnabledFeatures::setCompositeOpaqueScrollersEnabled(true);
 
     setBodyInnerHTML(
         "<style>"
         "#scroller { overflow: scroll; height: 200px; width: 200px; background: white local content-box; border: 10px solid rgba(0, 255, 0, 0.5); }"
         "#scrolled { height: 300px; }"
@@ skipping 84 matching lines @@
     paintLayer = toLayoutBoxModelObject(scroller->layoutObject())->layer();
     ASSERT_TRUE(paintLayer);
     EXPECT_TRUE(paintLayer->needsCompositedScrolling());
     EXPECT_TRUE(paintLayer->graphicsLayerBacking());
     ASSERT_TRUE(paintLayer->graphicsLayerBackingForScrolling());
     EXPECT_TRUE(paintLayer->graphicsLayerBackingForScrolling()->contentsOpaque());
 }
 
 
 }