Don't always fully repaint on viewport resize

Source/core/rendering/RenderView.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 167 matching lines @@
 void RenderView::checkLayoutState()
 {
     if (!RuntimeEnabledFeatures::repaintAfterLayoutEnabled()) {
         ASSERT(layoutDeltaMatches(LayoutSize()));
     }
     ASSERT(!m_layoutStateDisableCount);
     ASSERT(!m_layoutState->next());
 }
 #endif
 
+bool RenderView::shouldDoFullRepaintForNextLayout() const
+{
+    // It's hard to predict here which of full repaint or per-descendant repaint costs less.
+    // For vertical writing mode or width change it's more likely that per-descendant repaint
+    // eventually turns out to be full repaint but with the cost to handle more layout states
+    // and discrete repaint rects, so marking full repaint here is more likely to cost less.
+    // Otherwise, per-descendant repaint is more likely to avoid unnecessary full repaints.
+
+    if (shouldUsePrintingLayout())
+        return true;
+
+    if (!style()->isHorizontalWritingMode() || width() != viewWidth())
+        return true;
+
+    if (height() != viewHeight()) {
+        if (RenderObject* backgroundRenderer = this->backgroundRenderer()) {
+            if (backgroundRenderer->style()->backgroundImageNeedsFullRepaintOnContainerHeightChange())
+                return true;
+        }
+    }
+
+    return false;
+}
+
 void RenderView::layout()
 {
     if (!document().paginated())
         setPageLogicalHeight(0);
 
     if (shouldUsePrintingLayout())
         m_minPreferredLogicalWidth = m_maxPreferredLogicalWidth = logicalWidth();
 
     SubtreeLayoutScope layoutScope(*this);
 
@@ skipping 582 matching lines @@
 
 IntRect RenderView::unscaledDocumentRect() const
 {
     LayoutRect overflowRect(layoutOverflowRect());
     flipForWritingMode(overflowRect);
     return pixelSnappedIntRect(overflowRect);
 }
 
 bool RenderView::rootBackgroundIsEntirelyFixed() const
 {
-    RenderObject* rootObject = document().documentElement() ? document().documentElement()->renderer() : 0;
-    if (!rootObject)
-        return false;
+    if (RenderObject* backgroundRenderer = this->backgroundRenderer())
+        return backgroundRenderer->hasEntirelyFixedBackground();
+    return false;
+}
 
-    RenderObject* rootRenderer = rootObject->rendererForRootBackground();
-    return rootRenderer->hasEntirelyFixedBackground();
+RenderObject* RenderView::backgroundRenderer() const
+{
+    if (Element* documentElement = document().documentElement()) {
+        if (RenderObject* rootObject = documentElement->renderer())
+            return rootObject->rendererForRootBackground();
+    }
+    return 0;
 }
 
 LayoutRect RenderView::backgroundRect(RenderBox* backgroundRenderer) const
 {
     if (!hasColumns())
         return unscaledDocumentRect();
 
     ColumnInfo* columnInfo = this->columnInfo();
     LayoutRect backgroundRect(0, 0, columnInfo->desiredColumnWidth(), columnInfo->columnHeight() * columnInfo->columnCount());
     if (!isHorizontalWritingMode())
@@ skipping 157 matching lines @@
     return viewWidth(IncludeScrollbars) / scale;
 }
 
 double RenderView::layoutViewportHeight() const
 {
     float scale = m_frameView ? m_frameView->frame().pageZoomFactor() : 1;
     return viewHeight(IncludeScrollbars) / scale;
 }
 
 } // namespace WebCore