Use recomputed interest rect only if it changed enough

third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 275 matching lines @@
     if (offset == m_offsetFromLayoutObject)
         return;
 
     m_offsetFromLayoutObject = offset;
 
     // If the compositing layer offset changes, we need to repaint.
     if (shouldSetNeedsDisplay == SetNeedsDisplay)
         setNeedsDisplay();
 }
 
-void GraphicsLayer::paint(GraphicsContext& context, const IntRect* clip)
+void GraphicsLayer::paint(GraphicsContext& context, const IntRect* interestRect)
 {
-    ASSERT(clip || RuntimeEnabledFeatures::slimmingPaintSynchronizedPaintingEnabled());
+    ASSERT(interestRect || RuntimeEnabledFeatures::slimmingPaintSynchronizedPaintingEnabled());
     ASSERT(drawsContent());
 
     if (!m_client)
         return;
     if (firstPaintInvalidationTrackingEnabled())
         m_debugInfo.clearAnnotatedInvalidateRects();
     incrementPaintCount();
 #ifndef NDEBUG
     if (m_paintController && contentsOpaque() && s_drawDebugRedFill) {
         FloatRect rect(FloatPoint(), size());
         if (!DrawingRecorder::useCachedDrawingIfPossible(context, *this, DisplayItem::DebugRedFill)) {
             DrawingRecorder recorder(context, *this, DisplayItem::DebugRedFill, rect);
             context.fillRect(rect, SK_ColorRED);
         }
     }
 #endif
-    m_client->paintContents(this, context, m_paintingPhase, clip);
+
+    IntRect newInterestRect;
+    if (RuntimeEnabledFeatures::slimmingPaintSynchronizedPaintingEnabled()) {
+        if (!interestRect) {
+            newInterestRect = m_client->computeInterestRect(this, m_previousInterestRect);
+            interestRect = &newInterestRect;
+        }
+        if (!m_client->needsRepaint() && !paintController()->cacheIsEmpty() && m_previousInterestRect == *interestRect) {
+            paintController()->createAndAppend<CachedDisplayItem>(*this, DisplayItem::CachedDisplayItemList);
+            return;
+        }
+    }
+
+    m_client->paintContents(this, context, m_paintingPhase, *interestRect);
     notifyFirstPaintToClient();
+    m_previousInterestRect = *interestRect;
 }
 
 void GraphicsLayer::notifyFirstPaintToClient()
 {
     if (!m_painted) {
         m_painted = true;
         m_client->notifyFirstPaint();
     }
     if (!m_textPainted && m_paintController->textPainted()) {
         m_textPainted = true;
@@ skipping 865 matching lines @@
 {
     if (!layer) {
         fprintf(stderr, "Cannot showGraphicsLayerTree for (nil).\n");
         return;
     }
 
     String output = layer->layerTreeAsText(blink::LayerTreeIncludesDebugInfo);
     fprintf(stderr, "%s\n", output.utf8().data());
 }
 #endif