Where an ancestor has overflow style, a fixed child needs to be composited.

Source/core/layout/compositing/CompositingRequirementsUpdater.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google 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
@@ skipping 205 matching lines @@
     CompositingReasons reasonsToComposite = CompositingReasonNone;
     CompositingReasons directReasons = m_compositingReasonFinder.directReasons(layer);
 
     // Video is special. It's the only DeprecatedPaintLayer type that can both have
     // DeprecatedPaintLayer children and whose children can't use its backing to render
     // into. These children (the controls) always need to be promoted into their
     // own layers to draw on top of the accelerated video.
     if (currentRecursionData.m_compositingAncestor && currentRecursionData.m_compositingAncestor->layoutObject()->isVideo())
         directReasons |= CompositingReasonVideoOverlay;
 
+    if (currentRecursionData.m_hasCompositedScrollingAncestor && layer->layoutObject()->styleRef().hasViewportConstrainedPosition())
+        directReasons |= CompositingReasonPositionFixed;
+
     if (compositor->canBeComposited(layer)) {
         reasonsToComposite |= directReasons;
 
         if (layer->isRootLayer() && compositor->rootShouldAlwaysComposite())
             reasonsToComposite |= CompositingReasonRoot;
 
         if (reasonsToComposite && layer->scrollsOverflow() && !layer->needsCompositedScrolling()) {
             // We will only set needsCompositedScrolling if we don't care about
             // the LCD text hit, we may be able to switch to the compositor
             // driven path if we're alread composited for other reasons and are
@@ skipping 203 matching lines @@
 
         descendantHas3DTransform |= anyDescendantHas3DTransform || layer->has3DTransform();
     }
 
     // At this point we have finished collecting all reasons to composite this layer.
     layer->setCompositingReasons(reasonsToComposite);
 
 }
 
 } // namespace blink