Use recomputed interest rect only if it changed enough

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 b8440dc009530511a66b0bc3962c79e57523656f..c644b99ab1534dd3ac4c540083b73ce681148d34 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -2256,30 +2256,35 @@ bool CompositedLayerMapping::interestRectChangedEnoughToRepaint(const IntRect& p
     return false;
 }
 
-void CompositedLayerMapping::paintContents(const GraphicsLayer* graphicsLayer, GraphicsContext& context, GraphicsLayerPaintingPhase graphicsLayerPaintingPhase, const IntRect* interestRect) const
+IntRect CompositedLayerMapping::paintContents(const GraphicsLayer* graphicsLayer, GraphicsContext& context, GraphicsLayerPaintingPhase graphicsLayerPaintingPhase, const IntRect* interestRect) const
 {
     IntRect defaultInterestRect;
+    IntRect previousInterestRect = graphicsLayer->previousInterestRect();

[chrishtr, 2015/11/16 23:29:40]

If there were a method computeInterestRect and
interestRectChangedEnoughToRepaint on GraphicsLayerClient, all this logic could
be moved into GraphicsLayer, then this method would not have to take an optional
interestRect argument and return a value.

[Xianzhu, 2015/11/17 00:42:11]

Done.

     if (RuntimeEnabledFeatures::slimmingPaintSynchronizedPaintingEnabled()) {
         if (!interestRect) {
             if (graphicsLayer == m_graphicsLayer || graphicsLayer == m_squashingLayer)
                 defaultInterestRect = computeInterestRect(graphicsLayer, m_owningLayer.layoutObject());
             else
                 defaultInterestRect = enclosingIntRect(FloatRect(FloatPoint(), graphicsLayer->size()));
-            interestRect = &defaultInterestRect;
+
+            if (interestRectChangedEnoughToRepaint(previousInterestRect, defaultInterestRect, expandedIntSize(graphicsLayer->size())))
+                interestRect = &defaultInterestRect;
+            else
+                interestRect = &previousInterestRect;
         }
 
         if (!m_owningLayer.needsRepaint()
             && !context.paintController().cacheIsEmpty()
-            && !interestRectChangedEnoughToRepaint(m_previousPaintInterestRect, *interestRect, expandedIntSize(graphicsLayer->size()))) {
+            && previousInterestRect == *interestRect) {
             context.paintController().createAndAppend<CachedDisplayItem>(*this, DisplayItem::CachedDisplayItemList);
-            return;
+            return previousInterestRect;
         }
-
-        m_previousPaintInterestRect = *interestRect;
     }
 
     ASSERT(interestRect);
     paintContentsInternal(graphicsLayer, context, graphicsLayerPaintingPhase, *interestRect);
+
+    return *interestRect;
 }
 
 void CompositedLayerMapping::paintContentsInternal(const GraphicsLayer* graphicsLayer, GraphicsContext& context, GraphicsLayerPaintingPhase graphicsLayerPaintingPhase, const IntRect& interestRect) const