Composite sticky-positioned elements when they have composited descendants

third_party/WebKit/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
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  *    notice, this list of conditions and the following disclaimer.
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@@ skipping 172 matching lines @@
       subtreeReasons |= CompositingReasonIsolateCompositedDescendants;
     }
 
     // FIXME: This should move into
     // CompositingReasonFinder::potentialCompositingReasonsFromStyle, but theres
     // a poor interaction with LayoutTextControlSingleLine, which sets this
     // hasOverflowClip directly.
     if (layer->layoutObject().hasClipRelatedProperty())
       subtreeReasons |= CompositingReasonClipsCompositingDescendants;
 
-    if (layer->layoutObject().style()->position() == EPosition::kFixed)
-      subtreeReasons |= CompositingReasonPositionFixedWithCompositedDescendants;
+    // TODO(smcgruer): Use hasStickyConstrainedPosition() from flackr's patch
+    if (layer->layoutObject().style()->position() == EPosition::kFixed ||
+        layer->layoutObject().style()->position() == EPosition::kSticky) {
+      subtreeReasons |=
+          CompositingReasonPositionFixedOrStickyWithCompositedDescendants;

[flackr, 2017/03/23 17:30:58]

Will this cause the scroller to be composited too? If not we need to test
composited sticky in non-composited overflow scrollers.

[smcgruer, 2017/03/24 18:28:28]

It will not (based on testing http://output.jsbin.com/kahixo), but that
situation should work. Added layout test for it - do you think it needs a
unittest too?

[flackr, 2017/04/03 17:27:59]

No I think just having the layout test is fine.

+    }
   }
 
   // A layer with preserve-3d or perspective only needs to be composited if
   // there are descendant layers that will be affected by the preserve-3d or
   // perspective.
   if (has3DTransformedDescendants) {
     subtreeReasons |= layer->potentialCompositingReasonsFromStyle() &
                       CompositingReasonCombo3DDescendants;
   }
 
@@ skipping 333 matching lines @@
     descendantHas3DTransform |=
         anyDescendantHas3DTransform || layer->has3DTransform();
   }
 
   // At this point we have finished collecting all reasons to composite this
   // layer.
   layer->setCompositingReasons(reasonsToComposite);
 }
 
 }  // namespace blink