Composite sticky-positioned elements when they have composited descendants

third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
index aca28342b55b0d8d63095c2406bf06611a14e85a..ea03f368d524ff13961665f3d404bc172ad41e3a 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
@@ -190,8 +190,12 @@ static CompositingReasons subtreeReasonsForCompositing(
     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