Use PositionedMovementLayout for changes to positioned elements' margins

Source/core/rendering/style/RenderStyle.cpp

Index: Source/core/rendering/style/RenderStyle.cpp
diff --git a/Source/core/rendering/style/RenderStyle.cpp b/Source/core/rendering/style/RenderStyle.cpp
index b6db98ebcf328be94f64c9ecfebe286c9a719587..5bc10a9b66565f176cd14ab56e9b68093e850b85 100644
--- a/Source/core/rendering/style/RenderStyle.cpp
+++ b/Source/core/rendering/style/RenderStyle.cpp
@@ -385,6 +385,13 @@ StyleDifference RenderStyle::visualInvalidationDiff(const RenderStyle& other) co
     if (!diff.needsFullLayout() && diffNeedsFullLayout(other))
         diff.setNeedsFullLayout();
 
+    if (!diff.needsFullLayout() && surround->margin != other.surround->margin) {
+        if (position() == AbsolutePosition || position() == FixedPosition)

[Julien - ping for review, 2014/09/23 19:53:51]

Shouldn't we use position() != StaticPosition here like we do below?

Also wouldn't it make sense to refactor the code to handle this case right below
too as the conditions are pretty similar (surround->offset vs surround->margin)?

[rhogan, 2014/09/23 20:08:27]

We can't include RelativePosition here because their margins collapse - they do
need a layout. I should add a comment.
Yes it looked tempting to me too, and I tried it, but it got messy because of
the consideration above - and then I thought 'Maybe the next optimization we
find to add here will suggest a good refactoring, but for now I think I would be
making two straightforward cases look like one big unclear one.'

+            diff.setNeedsPositionedMovementLayout();
+        else
+            diff.setNeedsFullLayout();
+    }
+
     if (!diff.needsFullLayout() && position() != StaticPosition && surround->offset != other.surround->offset) {
         // Optimize for the case where a positioned layer is moving but not changing size.
         if (positionedObjectMovedOnly(surround->offset, other.surround->offset, m_box->width()))
@@ -603,9 +610,6 @@ bool RenderStyle::diffNeedsFullLayout(const RenderStyle& other) const
         return true;
 
     if (surround.get() != other.surround.get()) {
-        if (surround->margin != other.surround->margin)
-            return true;
-
         if (surround->padding != other.surround->padding)
             return true;
     }