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 97e85edde618dccb16c4f0300176fbc138f6dae0..0277233a3633b3b3a6d194dcfb416170a4d9821c 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, 2013/12/18 00:48:20]

Why is moving this outside the loop correct? Before we would have added this to
each containing block, now we only add it to the last one.

[Xianzhu, 2013/12/18 20:22:45]

I did some research about this (crbug.com/327815) and found the following facts:
1) Previously addPercentHeightDescendant is called for each percent height
grand-child or lower descendant child, but not for direct child. This was
intentional when the change was made because layout of direct children doesn't
depend on the percentHeightDescendant map;
2) It's unnecessary to add the intermediate containing blocks which don't affect
the height of the percent-height-descendant, because the percentage is only
relative to the containing block after the loop;
3) When the percentHeightDescendant map is used, actually layout is scheduled
for each containing block from the descendant up to the containing block which
is changing height.

For my change, a block needs to know all of its descendants (including direct
children) whose layout may be affected by the height of the block, so I added an
addPercentHeightDescendent() call after the loop:

while (...) {
  ...
  cb->addPercentHeightDescendant(this);
  cb = cb->containingBlock();
}
cb->addPercentHeightDescendent(this);

Because of above 2), I removed the call in the loop.

 
     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())

[ojan, 2013/12/18 00:48:20]

Can the containingBlock really not be a renderblock?

[Xianzhu, 2013/12/18 20:22:45]

Right. I encountered this when running layout tests. The height of a
percent-height out-of-flow-positioned RenderBox may relative to the height of a
RenderInline container. For example, in
LayoutTests/fast/replaced/absolute-position-percentage-height.html:

  <span style="position: relative">
    <img style="position: absolute; width: 50px; height:
100%><br><br><br><br><br>
  </span>

The check was not required before the change because addPercentHeightDescendant
was never called against the RenderInline container.

                 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