Skip PositionedMovementLayout when descendants depend on element's height

Source/core/rendering/RenderBlock.cpp

Index: Source/core/rendering/RenderBlock.cpp
diff --git a/Source/core/rendering/RenderBlock.cpp b/Source/core/rendering/RenderBlock.cpp
index bd49f4ad3547517a216814008da89d5912a61957..792d6442ccb5c40f75cc6eea8529fb1de1a6946a 100644
--- a/Source/core/rendering/RenderBlock.cpp
+++ b/Source/core/rendering/RenderBlock.cpp
@@ -3878,6 +3878,28 @@ bool RenderBlock::recalcOverflowAfterStyleChange()
     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()

[Julien - ping for review, 2015/01/06 14:35:09]

I can understand why only blocks use this function, however boxes can be
positioned (e.g. table row / row group) so it made sense for the function to be
on RenderBox. Combined with the history churn, I would argue to leave the
function on RenderBox.

[rhogan, 2015/01/06 21:43:45]

The other reason I moved it is that hasPercentageHeightDescendants() is
RenderBlock-only. I think the two together make it right to move. What do you
think?

[Julien - ping for review, 2015/01/07 09:41:39]

I think we have no other option then as hasPercentageHeightDescendants() only
makes sense for RenderBlocks (it's only meaningful for containing blocks). What
that means is that positioned movement only is a block concept then, which we
are probably fine with.

+{
+    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();
+    if (hasPercentHeightDescendants() && oldHeight != logicalHeight())

[Julien - ping for review, 2015/01/06 14:35:09]

The percentage height descendant map is filled during full layout but I think it
works to use it here as we should do a full layout when it is dirtied.

+        return false;
+    return true;

[Julien - ping for review, 2015/01/06 14:35:09]

The past 3 lines can be combined into a single line, which I find more readable.
If needed you can add a comment that we only care about the logical height if we
have percentage height descendants.

return !hasPercentHeightDescendant() || oldHeight == logicalHeight();

+}
+
 #if ENABLE(ASSERT)
 void RenderBlock::checkPositionedObjectsNeedLayout()
 {