Disable layout optimization when column height may be non-uniform.

third_party/WebKit/Source/core/layout/LayoutBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc.
  *               All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 4702 matching lines @@
       pageRemainingLogicalHeightForOffset(logicalTop, AssociateWithLatterPage);
   if (spaceLeft < logicalHeight)
     child.setOffsetToNextPage(spaceLeft);
 }
 
 bool LayoutBox::childNeedsRelayoutForPagination(const LayoutBox& child) const {
   // TODO(mstensho): Should try to get this to work for floats too, instead of
   // just marking and bailing here.
   if (child.isFloating())
     return true;
+  const LayoutFlowThread* flowThread = child.flowThreadContainingBlock();
   LayoutUnit logicalTop = child.logicalTop();
   // Figure out if we really need to force re-layout of the child. We only need
   // to do this if there's a chance that we need to recalculate pagination
   // struts inside.
   if (LayoutUnit pageLogicalHeight = pageLogicalHeightForOffset(logicalTop)) {
     LayoutUnit logicalHeight = child.logicalHeightWithVisibleOverflow();
     LayoutUnit remainingSpace = pageRemainingLogicalHeightForOffset(
         logicalTop, AssociateWithLatterPage);
     if (child.offsetToNextPage()) {
       // We need to relayout unless we're going to break at the exact same
       // location as before.
       if (child.offsetToNextPage() != remainingSpace)
         return true;
+      // If column height isn't guaranteed to be uniform, we have no way of
+      // telling what has happened after the first break.
+      if (flowThread && flowThread->mayHaveNonUniformPageLogicalHeight())
+        return true;
     } else if (logicalHeight > remainingSpace) {
       // Last time we laid out this child, we didn't need to break, but now we
       // have to. So we need to relayout.
       return true;
     }
   } else if (child.offsetToNextPage()) {
     // This child did previously break, but it won't anymore, because we no
     // longer have a known fragmentainer height.
     return true;
   }
 
   // It seems that we can skip layout of this child, but we need to ask the flow
   // thread for permission first. We currently cannot skip over objects
   // containing column spanners.
-  LayoutFlowThread* flowThread = child.flowThreadContainingBlock();
   return flowThread && !flowThread->canSkipLayout(child);
 }
 
 void LayoutBox::markChildForPaginationRelayoutIfNeeded(
     LayoutBox& child,
     SubtreeLayoutScope& layoutScope) {
   DCHECK(!child.needsLayout());
   LayoutState* layoutState = view()->layoutState();
 
   if (layoutState->paginationStateChanged() ||
@@ skipping 879 matching lines @@
   LayoutRect rect = frameRect();
 
   LayoutBlock* block = containingBlock();
   if (block)
     block->adjustChildDebugRect(rect);
 
   return rect;
 }
 
 }  // namespace blink