Avoid layout/full-repaint on view height change if possible

Source/core/rendering/RenderBox.cpp

Index: Source/core/rendering/RenderBox.cpp
diff --git a/Source/core/rendering/RenderBox.cpp b/Source/core/rendering/RenderBox.cpp
index be4374f0f8c0a096b77b127b464ca3e2354c2699..58cd97a14c77820bbaebd2c5008740c648d97aba 100644
--- a/Source/core/rendering/RenderBox.cpp
+++ b/Source/core/rendering/RenderBox.cpp
@@ -2868,8 +2868,8 @@ LayoutUnit RenderBox::computePercentageLogicalHeight(const Length& height) const
         skippedAutoHeightContainingBlock = true;
         containingBlockChild = cb;
         cb = cb->containingBlock();
-        cb->addPercentHeightDescendant(const_cast<RenderBox*>(this));
     }
+    cb->addPercentHeightDescendant(const_cast<RenderBox*>(this));

[ojan, 2014/01/03 02:04:26]

I'm still not sure this is correct. For example, the containingBlock can change
if this is a position:absolute and one of it's ancestors gets set to
position:relative dynamically. Then we will be in a situation where don't know
if there's a percentage height descendant that we need to update and
dirtyForLayoutFromPercentageHeightDescendants will do the wrong thing.

I'm not sure. Maybe I'm wrong.

[Xianzhu, 2014/01/03 20:46:24]

Actually percentHeightDescendants set doesn't include positioned descendants
whose heights are calculated in another path than
RenderBox::computePercentageLogicalHeight(). Non-positioned boxes won't change
containing block in this way. I'm investigating this further to make sure.
Perhaps I need to add some asserts.

 
     RenderStyle* cbstyle = cb->style();
 
@@ -3034,10 +3034,10 @@ LayoutUnit RenderBox::computeReplacedLogicalHeightUsing(Length logicalHeight) co
         case Calculated:
         {
             RenderObject* cb = isOutOfFlowPositioned() ? container() : containingBlock();
-            while (cb->isAnonymous()) {
+            while (cb->isAnonymous())
                 cb = cb->containingBlock();
+            if (cb->isRenderBlock())
                 toRenderBlock(cb)->addPercentHeightDescendant(const_cast<RenderBox*>(this));
-            }
 
             // FIXME: This calculation is not patched for block-flow yet.
             // https://bugs.webkit.org/show_bug.cgi?id=46500