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 182 matching lines @@
     if (shouldUsePrintingLayout()) {
         m_minPreferredLogicalWidth = m_maxPreferredLogicalWidth = logicalWidth();
         setShouldDoFullRepaintAfterLayout(true);
     }
 
     SubtreeLayoutScope layoutScope(*this);
 
     // Use calcWidth/Height to get the new width/height, since this will take the full page zoom factor into account.
     bool relayoutChildren = !shouldUsePrintingLayout() && (!m_frameView || width() != viewWidth() || height() != viewHeight());
     if (relayoutChildren) {
-        // FIXME: Should not always fully repaint on resize.
-        setShouldDoFullRepaintAfterLayout(true);
+        // 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 (!style()->isHorizontalWritingMode() || width() != viewWidth()) {
+            setShouldDoFullRepaintAfterLayout(true);
+        } else if (RenderObject* backgroundRenderer = this->backgroundRenderer()) {
+            if (backgroundRenderer->style()->backgroundImageNeedsFullRepaintOnContainerHeightChange())
+                setShouldDoFullRepaintAfterLayout(true);
+        }

[esprehn, 2014/05/08 20:42:03]

Does this work if repaint-after-layout is turned off?

[Xianzhu, 2014/05/08 22:31:08]

Yes. This is also based on https://codereview.chromium.org/270663003/ which
repaints the RenderView in the case when RAL is not enabled.

 
         layoutScope.setChildNeedsLayout(this);
         for (RenderObject* child = firstChild(); child; child = child->nextSibling()) {
             if (child->isSVGRoot())
                 continue;
 
             if ((child->isBox() && toRenderBox(child)->hasRelativeLogicalHeight())
                     || child->style()->logicalHeight().isPercent()
                     || child->style()->logicalMinHeight().isPercent()
                     || child->style()->logicalMaxHeight().isPercent())
@@ skipping 570 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 150 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