Skip PositionedMovementLayout when descendants depend on element's height

Source/core/rendering/RenderBlock.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2010. 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
@@ skipping 3860 matching lines @@
 
     LayoutUnit oldClientAfterEdge = hasRenderOverflow() ? m_overflow->layoutClientAfterEdge() : clientLogicalBottom();
     computeOverflow(oldClientAfterEdge, true);
 
     if (hasOverflowClip())
         layer()->scrollableArea()->updateAfterOverflowRecalc();
 
     return !hasOverflowClip();
 }
 
+// Called when a positioned object moves but doesn't necessarily change size.  A simplified layout is attempted
+// that just updates the object's position. If the size does change, the object remains dirty.
+bool RenderBlock::tryLayoutDoingPositionedMovementOnly()
+{
+    LayoutUnit oldWidth = logicalWidth();
+    LogicalExtentComputedValues computedValues;
+    logicalExtentAfterUpdatingLogicalWidth(logicalTop(), computedValues);
+    // If we shrink to fit our width may have changed, so we still need full layout.
+    if (oldWidth != computedValues.m_extent)
+        return false;
+    setLogicalWidth(computedValues.m_extent);
+    setLogicalLeft(computedValues.m_position);
+    setMarginStart(computedValues.m_margins.m_start);
+    setMarginEnd(computedValues.m_margins.m_end);
+
+    LayoutUnit oldHeight = logicalHeight();
+    updateLogicalHeight();
+    return !hasPercentHeightDescendants() || oldHeight == logicalHeight();
+}
+
 #if ENABLE(ASSERT)
 void RenderBlock::checkPositionedObjectsNeedLayout()
 {
     if (!gPositionedDescendantsMap)
         return;
 
     if (TrackedRendererListHashSet* positionedDescendantSet = positionedObjects()) {
         TrackedRendererListHashSet::const_iterator end = positionedDescendantSet->end();
         for (TrackedRendererListHashSet::const_iterator it = positionedDescendantSet->begin(); it != end; ++it) {
             RenderBox* currBox = *it;
@@ skipping 22 matching lines @@
 void RenderBlock::showLineTreeAndMark(const InlineBox* markedBox1, const char* markedLabel1, const InlineBox* markedBox2, const char* markedLabel2, const RenderObject* obj) const
 {
     showRenderObject();
     for (const RootInlineBox* root = firstRootBox(); root; root = root->nextRootBox())
         root->showLineTreeAndMark(markedBox1, markedLabel1, markedBox2, markedLabel2, obj, 1);
 }
 
 #endif
 
 } // namespace blink