Allow subsequence caching for SVG roots that have PaintLayers.

third_party/WebKit/Source/core/paint/PaintLayerPainter.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/PaintLayerPainter.h"
 
 #include "core/frame/LocalFrame.h"
 #include "core/layout/LayoutView.h"
 #include "core/paint/ClipPathClipper.h"
 #include "core/paint/FilterPainter.h"
@@ skipping 133 matching lines @@
   // Don't create subsequence during special painting to avoid cache conflict
   // with normal painting.
   if (paintingInfo.getGlobalPaintFlags() & GlobalPaintFlattenCompositingLayers)
     return false;
   if (paintFlags &
       (PaintLayerPaintingRootBackgroundOnly |
        PaintLayerPaintingOverlayScrollbars | PaintLayerUncachedClipRects))
     return false;
 
   // Create subsequence for only stacking contexts whose painting are atomic.
-  if (!paintLayer.stackingNode()->isStackingContext())
+  if (!paintLayer.stackingNode()->isStackingContext() &&
+      !paintLayer.layoutObject()->isSVGRoot())

[wkorman, 2016/11/12 00:05:51]

Consider adding comment above this block re: why we allow for SVGRoot here and
below.

[chrishtr, 2016/11/12 00:15:20]

Done.

     return false;
 
   // The layer doesn't have children. Subsequence caching is not worth because
   // normally the actual painting will be cheap.
   if (!PaintLayerStackingNodeIterator(*paintLayer.stackingNode(), AllChildren)
-           .next())
+           .next() &&
+      !paintLayer.layoutObject()->isSVGRoot())
     return false;
 
   // When in FOUC-avoidance mode, don't cache any subsequences, to avoid having
   // to invalidate all of them when leaving this mode. There is an early-out in
   // BlockPainter::paintContents that may result in nothing getting painted in
   // this mode, in addition to early-out logic in PaintLayerPainter.
   if (paintLayer.layoutObject()->document().didLayoutWithPendingStylesheets())
     return false;
 
   return true;
@@ skipping 899 matching lines @@
 
   PaintLayerPaintingInfo paintingInfo(&m_paintLayer,
                                       LayoutRect(enclosingIntRect(damageRect)),
                                       paintFlags, LayoutSize());
   paintLayer(context, paintingInfo, PaintLayerPaintingOverlayScrollbars);
 
   m_paintLayer.setContainsDirtyOverlayScrollbars(false);
 }
 
 }  // namespace blink