[CSS Grid Layout] Grid items must set a new formatting context.

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 5f51066734ecbe40ef68acca735f517b9442cb21..d981d50a8b59cc0e4a79eaa34ef46e213502cc8c 100644
--- a/Source/core/rendering/RenderBlock.cpp
+++ b/Source/core/rendering/RenderBlock.cpp
@@ -1260,7 +1260,7 @@ bool RenderBlock::isSelfCollapsingBlock() const
     // We should be able to give a quick answer if the box is a relayout boundary.
     // Being a relayout boundary implies a block formatting context, and also
     // our internal layout shouldn't affect our container in any way.
-    if (createsBlockFormattingContext())
+    if (createsNewFormattingContext())
         return false;
 
     // Placeholder elements are not laid out until the dimensions of their parent text control are known, so they
@@ -1518,10 +1518,11 @@ void RenderBlock::addVisualOverflowFromTheme()
     addVisualOverflow(inflatedRect);
 }
 
-bool RenderBlock::createsBlockFormattingContext() const
+bool RenderBlock::createsNewFormattingContext() const
 {
     return isInlineBlockOrInlineTable() || isFloatingOrOutOfFlowPositioned() || hasOverflowClip() || isFlexItemIncludingDeprecated()
-        || style()->specifiesColumns() || isRenderFlowThread() || isTableCell() || isTableCaption() || isFieldset() || isWritingModeRoot() || isDocumentElement() || style()->columnSpan();
+        || style()->specifiesColumns() || isRenderFlowThread() || isTableCell() || isTableCaption() || isFieldset() || isWritingModeRoot()
+        || isDocumentElement() || style()->columnSpan() || isGridItem();
 }
 
 void RenderBlock::updateBlockChildDirtyBitsBeforeLayout(bool relayoutChildren, RenderBox* child)