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

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

I actually feel like we should call
CompositingReasonFinder::requiresCompositingForScrollDependentPosition instead,
since this is trying to duplicate the checks for sticky / fixed promotion there.

[smcgruer, 2017/04/03 19:59:53]

That would slightly the existing fixed behavior here though. It would add the
opaque-fixed-pos check and the 'layer->fixedToViewport() &&
m_layoutView.frameView()->isScrollable()' check. Are we sure that is desirable?
(I have no idea at first glance :).)

[flackr, 2017/04/07 18:50:43]

fixedToViewport doesn't check for opaqueness, just whether the fixed position
element is contained (scrolls) or not (fixed). Since it doesn't move with
respect to the scrolling contents if contained it makes sense not to promote it
as we're doing with sticky.

[flackr, 2017/04/09 04:07:02]

Sorry, meant to say we call it with ignoreLCDText = true which won't consider
opaqueness.

[smcgruer, 2017/04/21 15:17:32]

Done.

+    if (layer->layoutObject().style()->position() == EPosition::kFixed ||
+        layer->layoutObject().style()->position() == EPosition::kSticky) {
+      subtreeReasons |=
+          CompositingReasonPositionFixedOrStickyWithCompositedDescendants;
+    }
   }
 
   // A layer with preserve-3d or perspective only needs to be composited if