[css-grid] Fix simplified layout of positioned grid items

third_party/WebKit/Source/core/layout/LayoutGrid.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 457 matching lines @@
     computeTrackSizesForDefiniteSize(ForColumns, sizingData,
                                      availableSpaceForColumns);
     computeTrackSizesForDefiniteSize(ForRows, sizingData,
                                      availableSpaceForRows);
   }
 }
 
 void LayoutGrid::layoutBlock(bool relayoutChildren) {
   ASSERT(needsLayout());
 
-  if (!relayoutChildren && simplifiedLayout())
+  // We cannot perform a simplifiedLayout() on a dirty grid that
+  // has positioned items to be laid out.
+  if (!relayoutChildren && (!m_gridIsDirty || !posChildNeedsLayout()) &&
+      simplifiedLayout())
     return;
 
   SubtreeLayoutScope layoutScope(*this);
 
   {
     // LayoutState needs this deliberate scope to pop before updating scroll
     // information (which may trigger relayout).
     LayoutState state(*this);
 
     LayoutSize previousSize = size();
@@ skipping 2947 matching lines @@
   if (!m_gridItemArea.isEmpty())
     GridPainter(*this).paintChildren(paintInfo, paintOffset);
 }
 
 bool LayoutGrid::cachedHasDefiniteLogicalHeight() const {
   SECURITY_DCHECK(m_hasDefiniteLogicalHeight);
   return m_hasDefiniteLogicalHeight.value();
 }
 
 }  // namespace blink