[Reland] Correct fixed-position recording for Slimming Paint

Source/core/layout/LayoutBoxModelObject.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 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 206 matching lines @@
     if (layer()) {
         // FIXME: Ideally we shouldn't need this setter but we can't easily infer an overflow-only layer
         // from the style.
         layer()->setLayerType(type);
 
         layer()->styleChanged(diff, oldStyle);
         if (hadLayer && layer()->isSelfPaintingLayer() != layerWasSelfPainting)
             setChildNeedsLayout();
     }
 
+    // Fixed-position is painted using transform. In the case that the object
+    // gets the same layout after changing position property, although no
+    // re-raster (rect-based invalidation) is needed, display items should
+    // still update their paint offset.
+    if (RuntimeEnabledFeatures::slimmingPaintEnabled() && oldStyle) {
+        bool newStyleIsFixedPosition = style()->position() == FixedPosition;
+        bool oldStyleIsFixedPosition = oldStyle->position() == FixedPosition;
+        if (newStyleIsFixedPosition != oldStyleIsFixedPosition)
+            invalidateDisplayItemClientForNonCompositingDescendants();
+    }
+
     if (FrameView *frameView = view()->frameView()) {
         bool newStyleIsViewportConstained = style()->hasViewportConstrainedPosition();
         bool oldStyleIsViewportConstrained = oldStyle && oldStyle->hasViewportConstrainedPosition();
         if (newStyleIsViewportConstained != oldStyleIsViewportConstrained) {
             if (newStyleIsViewportConstained && layer())
                 frameView->addViewportConstrainedObject(this);
             else
                 frameView->removeViewportConstrainedObject(this);
         }
     }
@@ skipping 676 matching lines @@
     ASSERT(!beforeChild || toBoxModelObject == beforeChild->parent());
     for (LayoutObject* child = startChild; child && child != endChild; ) {
         // Save our next sibling as moveChildTo will clear it.
         LayoutObject* nextSibling = child->nextSibling();
         moveChildTo(toBoxModelObject, child, beforeChild, fullRemoveInsert);
         child = nextSibling;
     }
 }
 
 } // namespace blink