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

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc.
  *               All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 386 matching lines @@
     return;
 
   if (oldStyle->gridColumnStart() == style()->gridColumnStart() &&
       oldStyle->gridColumnEnd() == style()->gridColumnEnd() &&
       oldStyle->gridRowStart() == style()->gridRowStart() &&
       oldStyle->gridRowEnd() == style()->gridRowEnd() &&
       oldStyle->order() == style()->order() &&
       oldStyle->hasOutOfFlowPosition() == style()->hasOutOfFlowPosition())
     return;
 
+  // Positioned items don't participate on the layout of the grid,
+  // so we don't need to mark the grid as dirty if they change positions.
+  if (oldStyle->hasOutOfFlowPosition() && style()->hasOutOfFlowPosition())
+    return;
+
   // It should be possible to not dirty the grid in some cases (like moving an
   // explicitly placed grid item).
   // For now, it's more simple to just always recompute the grid.
   toLayoutGrid(parent())->dirtyGrid();
 }
 
 void LayoutBox::updateScrollSnapMappingAfterStyleChange(
     const ComputedStyle* newStyle,
     const ComputedStyle* oldStyle) {
   SnapCoordinator* snapCoordinator = document().snapCoordinator();
@@ skipping 5221 matching lines @@
   LayoutRect rect = frameRect();
 
   LayoutBlock* block = containingBlock();
   if (block)
     block->adjustChildDebugRect(rect);
 
   return rect;
 }
 
 }  // namespace blink