[CSS Grid Layout] Avoid inserting grid items twice in the grid

Source/core/rendering/RenderGrid.cpp

Index: Source/core/rendering/RenderGrid.cpp
diff --git a/Source/core/rendering/RenderGrid.cpp b/Source/core/rendering/RenderGrid.cpp
index a1de68202177f4dafa8c7e86d0569ead6c56f4df..76756b0a11f9ffd3ec5f9cafea605d99064a92e1 100644
--- a/Source/core/rendering/RenderGrid.cpp
+++ b/Source/core/rendering/RenderGrid.cpp
@@ -207,6 +207,15 @@ void RenderGrid::addChild(RenderObject* newChild, RenderObject* beforeChild)
     if (gridIsDirty())
         return;
 
+    // If the new requested beforeChild is not one of our children is because it's wrapped by an anonymous container. In
+    // those cases addChild will be called twice for the newChild, one with the initial beforeChild and another one with
+    // its parent (the anonymous block container). See RenderBlock::addChildIgnoringAnonymousColumnBlocks()
+    if (beforeChild && beforeChild->parent() != this) {
+        ASSERT(beforeChild->parent()->isAnonymous());
+        dirtyGrid();

[Julien - ping for review, 2014/06/25 21:25:08]

Do we really need to invalidate the grid in this case if we assume we are going
to get called for the real children?

Also other renderers (tables, blocks) have to deal with similar issues and they
end up splitting the renderer in 2. I think we may need some similar logic here
(see splitAnonymousBoxesAroundChild).

[svillar, 2014/06/26 09:45:53]

Good point, should be safe to just return indeed.
Hmm let's take a look.

+        return;
+    }
+
     if (!newChild->isBox()) {
         dirtyGrid();
         return;