Repaint when background switches painting from/to scrolling contents layer.

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index 99a217ea891822244bcdcb4c8ef4925719edf1da..e6397348d169f507c61c3f8012dc8e5932042294 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -174,6 +174,7 @@ CompositedLayerMapping::CompositedLayerMapping(PaintLayer& layer)
     , m_isMainFrameLayoutViewLayer(false)
     , m_backgroundLayerPaintsFixedRootBackground(false)
     , m_scrollingContentsAreEmpty(false)
+    , m_backgroundPaintsOntoScrollingContentsLayer(false)
 {
     if (layer.isRootLayer() && layoutObject()->frame()->isMainFrame())
         m_isMainFrameLayoutViewLayer = true;
@@ -294,9 +295,24 @@ void CompositedLayerMapping::updateIsRootForIsolatedGroup()
     m_graphicsLayer->setIsRootForIsolatedGroup(isolate);
 }
 
+void CompositedLayerMapping::updateBackgroundPaintingLayer()

[chrishtr, 2016/08/24 23:35:32]

updateBackgroundPaintsOntoScrollingContentsLayer

[flackr, 2016/08/25 15:21:52]

Done.

+{
+    bool backgroundLayerPaintsOntoScrollingContentsLayer = shouldPaintBackgroundOntoScrollingContentsLayer();
+    if (backgroundLayerPaintsOntoScrollingContentsLayer != m_backgroundPaintsOntoScrollingContentsLayer) {
+        m_backgroundPaintsOntoScrollingContentsLayer = backgroundLayerPaintsOntoScrollingContentsLayer;
+        // If the background is no longer painted onto the scrolling contents
+        // layer the scrolling contents layer needs to be updated. If it is
+        // going to be painted onto the scrolling contents layer this update
+        // will be triggered by LayoutBoxModelObject::setBackingNeedsPaintInvalidationInRect
+        if (hasScrollingLayer() && !backgroundLayerPaintsOntoScrollingContentsLayer) {
+            m_scrollingContentsLayer->setNeedsDisplay();
+            m_owningLayer.setNeedsRepaint();
+        }
+    }
+}
+
 void CompositedLayerMapping::updateContentsOpaque()
 {
-    ASSERT(m_isMainFrameLayoutViewLayer || !m_backgroundLayer);
     if (isAcceleratedCanvas(layoutObject())) {
         // Determine whether the rendering context's external texture layer is opaque.
         CanvasRenderingContext* context = toHTMLCanvasElement(layoutObject()->node())->renderingContext();
@@ -312,7 +328,7 @@ void CompositedLayerMapping::updateContentsOpaque()
     } else {
         // For non-root layers, background is painted by the scrolling contents layer if all backgrounds
         // are background attachment local, otherwise background is painted by the primary graphics layer.
-        if (hasScrollingLayer() && shouldPaintBackgroundOntoScrollingContentsLayer()) {
+        if (hasScrollingLayer() && m_backgroundPaintsOntoScrollingContentsLayer) {
             // Backgrounds painted onto the foreground are clipped by the padding box rect.
             // TODO(flackr): This should actually check the entire overflow rect within the
             // scrolling contents layer but since we currently only trigger this for solid
@@ -324,6 +340,8 @@ void CompositedLayerMapping::updateContentsOpaque()
             // not opaque.
             m_graphicsLayer->setContentsOpaque(false);
         } else {
+            if (hasScrollingLayer())
+                m_scrollingContentsLayer->setContentsOpaque(false);

[chrishtr, 2016/08/24 23:35:32]

Is this technically an unrelated fix?

Is it tested?

[flackr, 2016/08/25 15:21:52]

No, this is necessary to remove contents opaque flag when we switch from having
a background painted onto the scrolling contents layer to not.
Yes, in the added test the scrolling contents layer remains opaque (and turns
cyan in debug and usually black in release) when the background is no longer
painted into it if we don't call setContentsOpaque.

             m_graphicsLayer->setContentsOpaque(m_owningLayer.backgroundIsKnownToBeOpaqueInRect(compositedBounds()));
         }
     }
@@ -769,6 +787,7 @@ void CompositedLayerMapping::updateGraphicsLayerGeometry(const PaintLayer* compo
     updateBackgroundColor();
     updateDrawsContent();
     updateElementIdAndCompositorMutableProperties();
+    updateBackgroundPaintingLayer();
     updateContentsOpaque();
     updateAfterPartResize();
     updateRenderingContext();
@@ -2461,7 +2480,7 @@ void CompositedLayerMapping::paintContents(const GraphicsLayer* graphicsLayer, G
         || graphicsLayer == m_childClippingMaskLayer.get()
         || graphicsLayer == m_scrollingContentsLayer.get()) {
 
-        bool paintRootBackgroundOntoScrollingContentsLayer = shouldPaintBackgroundOntoScrollingContentsLayer();
+        bool paintRootBackgroundOntoScrollingContentsLayer = m_backgroundPaintsOntoScrollingContentsLayer;
         DCHECK(!paintRootBackgroundOntoScrollingContentsLayer || (!m_backgroundLayer && !m_foregroundLayer));
         if (paintRootBackgroundOntoScrollingContentsLayer) {
             if (graphicsLayer == m_scrollingContentsLayer.get())