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

BUG=647169,645949
TEST=CompositedLayerMappingTest.BackgroundPaintedIntoGraphicsLayerIfNotCompositedScrolling
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2346883003
Cr-Commit-Position: refs/heads/master@{#419064}

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

@@ -137,11 +137,6 @@ bool LayoutBoxModelObject::hasLocalEquivalentBackground() const
     if (!style()->hasAutoClip())
         return false;
 
-    // TODO(flackr): We should be able to paint locally attached borders with a border-radius
-    // but these currently do not paint correctly. https://crbug.com/645949
-    if (style()->hasBorderRadius())
-        return false;
-
     // TODO(flackr): Remove this when box shadows are still painted correctly when painting
     // into the composited scrolling contents layer. https://crbug.com/646464
     if (style()->boxShadow())

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

@@ -300,7 +300,12 @@ void CompositedLayerMapping::updateIsRootForIsolatedGroup()
 
 void CompositedLayerMapping::updateBackgroundPaintsOntoScrollingContentsLayer()
 {
-    bool shouldPaintOntoScrollingContentsLayer = m_owningLayer.shouldPaintBackgroundOntoScrollingContentsLayer();
+    // We can only paint the background onto the scrolling contents layer if
+    // it would be visually correct and we are using composited scrolling meaning we
+    // have a scrolling contents layer to paint it into.
+    bool shouldPaintOntoScrollingContentsLayer =
+        m_owningLayer.canPaintBackgroundOntoScrollingContentsLayer()
+        && m_owningLayer.getScrollableArea()->usesCompositedScrolling();
     if (shouldPaintOntoScrollingContentsLayer != backgroundPaintsOntoScrollingContentsLayer()) {
         m_backgroundPaintsOntoScrollingContentsLayer = shouldPaintOntoScrollingContentsLayer;
         // If the background is no longer painted onto the scrolling contents

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

@@ -647,6 +647,25 @@ TEST_F(CompositedLayerMappingTest, ScrollingContentsAndForegroundLayerPaintingPh
     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)

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

@@ -2358,7 +2358,7 @@ GraphicsLayer* PaintLayer::graphicsLayerBackingForScrolling() const
     }
 }
 
-bool PaintLayer::shouldPaintBackgroundOntoScrollingContentsLayer() const
+bool PaintLayer::canPaintBackgroundOntoScrollingContentsLayer() const
 {
     return !isRootLayer()
         && scrollsOverflow()

third_party/WebKit/Source/core/paint/PaintLayer.h

@@ -438,7 +438,7 @@ public:
     // (i.e. the background can scroll with the content). When the background is also
     // opaque this allows us to composite the scroller even on low DPI as we can
     // draw with subpixel anti-aliasing.
-    bool shouldPaintBackgroundOntoScrollingContentsLayer() const;
+    bool canPaintBackgroundOntoScrollingContentsLayer() const;
     // NOTE: If you are using hasCompositedLayerMapping to determine the state of compositing for this layer,
     // (and not just to do bookkeeping related to the mapping like, say, allocating or deallocating a mapping),
     // then you may have incorrect logic. Use compositingState() instead.

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

@@ -1497,7 +1497,7 @@ static bool layerNeedsCompositedScrolling(PaintLayerScrollableArea::LCDTextMode
     // use integer scroll offsets locally when !layer->compositor()->preferCompositingToLCDTextEnabled(),
     // we do not check offsets accumulated from the root (including translates). crbug.com/644833
     bool backgroundSupportsLCDText = RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled()
-        && layer->shouldPaintBackgroundOntoScrollingContentsLayer()
+        && layer->canPaintBackgroundOntoScrollingContentsLayer()
         && layer->backgroundIsKnownToBeOpaqueInRect(toLayoutBox(layer->layoutObject())->paddingBoxRect());
     if (mode == PaintLayerScrollableArea::ConsiderLCDText
         && !layer->compositor()->preferCompositingToLCDTextEnabled()

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

@@ -8,7 +8,7 @@
 
 namespace blink {
 
-TEST_F(PaintLayerScrollableAreaTest, ShouldPaintBackgroundOntoScrollingContentsLayer)
+TEST_F(PaintLayerScrollableAreaTest, CanPaintBackgroundOntoScrollingContentsLayer)
 {
     document().frame()->settings()->setPreferCompositingToLCDTextEnabled(true);
     setBodyInnerHTML(
@@ -34,72 +34,67 @@ TEST_F(PaintLayerScrollableAreaTest, ShouldPaintBackgroundOntoScrollingContentsL
         "<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>"
+        "<div id='scroller16' 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"));
+    EXPECT_FALSE(canPaintBackgroundOntoScrollingContentsLayer("scroller1"));
 
     // #scroller2 cannot paint background into scrolling contents layer because it has a content-box clip without local attachment.
-    EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller2"));
+    EXPECT_FALSE(canPaintBackgroundOntoScrollingContentsLayer("scroller2"));
 
     // #scroller3 can paint background into scrolling contents layer.
-    EXPECT_TRUE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller3"));
+    EXPECT_TRUE(canPaintBackgroundOntoScrollingContentsLayer("scroller3"));
 
     // #scroller4 cannot paint background into scrolling contents layer because the background image is not locally attached.
-    EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller4"));
+    EXPECT_FALSE(canPaintBackgroundOntoScrollingContentsLayer("scroller4"));
 
     // #scroller5 can paint background into scrolling contents layer because both the image and color are locally attached.
-    EXPECT_TRUE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller5"));
+    EXPECT_TRUE(canPaintBackgroundOntoScrollingContentsLayer("scroller5"));
 
     // #scroller6 can paint background into scrolling contents layer because the image is locally attached and even though
     // the color is not, it is filled to the padding box so it will be drawn the same as a locally attached background.
-    EXPECT_TRUE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller6"));
+    EXPECT_TRUE(canPaintBackgroundOntoScrollingContentsLayer("scroller6"));
 
     // #scroller7 cannot paint background into scrolling contents layer because the color is filled to the content
     // box and we have padding so it is not equivalent to a locally attached background.
-    EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller7"));
+    EXPECT_FALSE(canPaintBackgroundOntoScrollingContentsLayer("scroller7"));
 
     // #scroller8 can paint background into scrolling contents layer because its border-box is equivalent to its
     // padding box since it has no border.
-    EXPECT_TRUE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller8"));
+    EXPECT_TRUE(canPaintBackgroundOntoScrollingContentsLayer("scroller8"));
 
     // #scroller9 can paint background into scrolling contents layer because its border is opaque so it completely
     // covers the background outside of the padding-box.
-    EXPECT_TRUE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller9"));
+    EXPECT_TRUE(canPaintBackgroundOntoScrollingContentsLayer("scroller9"));
 
     // #scroller10 cannot paint background into scrolling contents layer because its border is partially transparent
     // so the background must be drawn to the border-box edges.
-    EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller10"));
+    EXPECT_FALSE(canPaintBackgroundOntoScrollingContentsLayer("scroller10"));
 
     // #scroller11 can paint background into scrolling contents layer because its content-box is equivalent to its
     // padding box since it has no padding.
-    EXPECT_TRUE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller11"));
+    EXPECT_TRUE(canPaintBackgroundOntoScrollingContentsLayer("scroller11"));
 
     // #scroller12 cannot paint background into scrolling contents layer because it has padding so its content-box
     // is not equivalent to its padding-box.
-    EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller12"));
+    EXPECT_FALSE(canPaintBackgroundOntoScrollingContentsLayer("scroller12"));
 
     // #scroller13 cannot paint background into scrolling contents layer because it has a custom
     // scrollbar which the background may need to draw under.
-    EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller13"));
+    EXPECT_FALSE(canPaintBackgroundOntoScrollingContentsLayer("scroller13"));
 
     // #scroller14 can paint background into scrolling contents layer because the outline is drawn outside the
     // padding box.
-    EXPECT_TRUE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller14"));
+    EXPECT_TRUE(canPaintBackgroundOntoScrollingContentsLayer("scroller14"));
 
     // #scroller15 cannot paint background into scrolling contents layer because the outline is drawn inside
     // the padding box.
-    EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller15"));
+    EXPECT_FALSE(canPaintBackgroundOntoScrollingContentsLayer("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
+    // #scroller16 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"));
+    EXPECT_FALSE(canPaintBackgroundOntoScrollingContentsLayer("scroller16"));
 }
 
 TEST_F(PaintLayerScrollableAreaTest, OpaqueLayersPromoted)

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

@@ -17,10 +17,10 @@ public:
         : RenderingTest(SingleChildFrameLoaderClient::create())
     { }
 
-    bool shouldPaintBackgroundOntoScrollingContentsLayer(const char* elementId)
+    bool canPaintBackgroundOntoScrollingContentsLayer(const char* elementId)
     {
         PaintLayer* paintLayer = toLayoutBlock(getLayoutObjectByElementId(elementId))->layer();
-        return paintLayer->shouldPaintBackgroundOntoScrollingContentsLayer();
+        return paintLayer->canPaintBackgroundOntoScrollingContentsLayer();
     }
 
 private: