DCHECK that paint properties are never null

third_party/WebKit/Source/core/paint/FramePainter.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/FramePainter.h"
 
 #include "core/editing/markers/DocumentMarkerController.h"
 #include "core/fetch/MemoryCache.h"
 #include "core/frame/FrameView.h"
 #include "core/inspector/InspectorInstrumentation.h"
@@ skipping 76 matching lines @@
     DCHECK(!RuntimeEnabledFeatures::rootLayerScrollingEnabled());
     IntRect scrollViewDirtyRect = rect.m_rect;
     IntRect visibleAreaWithScrollbars(
         frameView().location(),
         frameView().visibleContentRect(IncludeScrollbars).size());
     scrollViewDirtyRect.intersect(visibleAreaWithScrollbars);
     scrollViewDirtyRect.moveBy(-frameView().location());
 
     Optional<ScopedPaintChunkProperties> scopedPaintChunkProperties;
     if (RuntimeEnabledFeatures::slimmingPaintV2Enabled()) {
-      if (TransformPaintPropertyNode* transform =
-              m_frameView->preTranslation()) {
+      if (const PropertyTreeState* contentsState =
+              m_frameView->totalPropertyTreeStateForContents()) {
+        // The scrollbar's property nodes are similar to the frame view's
+        // contents state but we want to exclude the content-specific. This
+        // prevents the scrollbars from scrolling, for example.
         PaintChunkProperties properties(
             context.getPaintController().currentPaintChunkProperties());
-        properties.transform = transform;
+        properties.transform = m_frameView->preTranslation();
+        properties.clip = m_frameView->contentClip()->parent();
+        properties.effect = contentsState->effect();
+        auto* scrollBarScroll = contentsState->scroll();

[szager1, 2016/11/14 05:34:04]

As far as I can tell, contentsState->scroll() should always be the same as
m_frameView->scroll().  Under what circumstance would that not be true
(necessitating this fall back logic)?

+        if (m_frameView->scroll())
+          scrollBarScroll = m_frameView->scroll()->parent();
+        properties.scroll = scrollBarScroll;
         scopedPaintChunkProperties.emplace(context.getPaintController(),
                                            *frameView().layoutView(),
                                            properties);
       }
     }
 
     TransformRecorder transformRecorder(
         context, *frameView().layoutView(),
         AffineTransform::translation(frameView().x(), frameView().y()));
 
@@ skipping 130 matching lines @@
 
   bar.paint(context, CullRect(rect));
 }
 
 const FrameView& FramePainter::frameView() {
   DCHECK(m_frameView);
   return *m_frameView;
 }
 
 }  // namespace blink