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 c070f17b691aaa55202e27bd01b0c065aeb2c101..b1c4aa02c5bcea8978bed491f1a0144bc04e4b36 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -173,6 +173,7 @@ 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;
@@ -293,9 +294,24 @@ void CompositedLayerMapping::updateIsRootForIsolatedGroup()
     m_graphicsLayer->setIsRootForIsolatedGroup(isolate);
 }
 
+void CompositedLayerMapping::updateBackgroundPaintingLayer()
+{
+    bool backgroundLayerPaintsOntoScrollingContentsLayer = shouldPaintBackgroundOntoScrollingContentsLayer();
+    if (backgroundLayerPaintsOntoScrollingContentsLayer != m_backgroundPaintsOntoScrollingContentsLayer) {

[chrishtr, 2016/08/24 20:50:35]

What if backgroundLayerPaintsOntoScrollingContentsLayer starts to become true?
Doesn't m_owningLayer
need to be cleared out then?

[flackr, 2016/08/24 21:01:10]

It does, but in that case it's handled by
LayoutBoxModelObject::setBackingNeedsPaintInvalidationInRect because when we
know we're painting the background into the scrolling contents layer we
invalidate both layers.

+        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) {

[chrishtr, 2016/08/24 00:33:26]

This is not the right place for this invalidation. Instead it should be done via
a style update on the PaintLayer, and use
the usual mechanisms. Does that not work?

[flackr, 2016/08/24 01:34:51]

Hmm, PaintLayer::styleDidChange could detect most of the reasons for not being
able to paint onto the scrolling contents layer but would it be able to tell
when hasNegativeZOrderList had changed? I'm not sure I understand why
invalidating here when the background moves between the background layer and
scrolling contents layer is different than the setNeedsDisplay calls that happen
in CompositedLayerMapping::updateScrollingLayerGeometry. Does the composited
layer mapping update happen too late to invalidate? 

+            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();
@@ -311,7 +327,7 @@ 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
@@ -323,6 +339,8 @@ void CompositedLayerMapping::updateContentsOpaque()
             // not opaque.
             m_graphicsLayer->setContentsOpaque(false);
         } else {
+            if (hasScrollingLayer())
+                m_scrollingContentsLayer->setContentsOpaque(false);
             m_graphicsLayer->setContentsOpaque(m_owningLayer.backgroundIsKnownToBeOpaqueInRect(compositedBounds()));
         }
     }
@@ -768,6 +786,7 @@ void CompositedLayerMapping::updateGraphicsLayerGeometry(const PaintLayer* compo
     updateBackgroundColor();
     updateDrawsContent();
     updateElementIdAndCompositorMutableProperties();
+    updateBackgroundPaintingLayer();
     updateContentsOpaque();
     updateAfterPartResize();
     updateRenderingContext();
@@ -2460,7 +2479,7 @@ 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())