Mark elements as viewport constrained when they become sticky positioned

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp
index 92b93ede5558f7ae7b2706123227a683fa50e9b6..673b4e6b45ff70fac5e0b59f1ea56e9e409dbcf0 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp
@@ -102,24 +102,31 @@ void CompositingInputsUpdater::updateRecursive(PaintLayer* layer,
   layer->updateAncestorOverflowLayer(info.lastOverflowClipLayer);
   if (info.lastOverflowClipLayer && layer->needsCompositingInputsUpdate() &&
       layer->layoutObject()->style()->position() == EPosition::kSticky) {
-    if (info.lastOverflowClipLayer != previousOverflowLayer &&
-        !RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
-      // Old ancestor scroller should no longer have these constraints.
-      ASSERT(!previousOverflowLayer ||
-             !previousOverflowLayer->getScrollableArea()
-                  ->stickyConstraintsMap()
-                  .contains(layer));
-
-      if (info.lastOverflowClipLayer->isRootLayer())
+    if (!RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
+      if (info.lastOverflowClipLayer != previousOverflowLayer) {
+        // Old ancestor scroller should no longer have these constraints.
+        DCHECK(!previousOverflowLayer ||
+               !previousOverflowLayer->getScrollableArea()
+                    ->stickyConstraintsMap()
+                    .contains(layer));
+
+        // If our old ancestor scroller was the root layer, we may not be
+        // viewport constrained anymore. This might be re-added below if the new

[flackr, 2017/02/22 14:45:42]

I don't think the case in this comment can happen. If the previous overflow
layer was the root layer and is not equal to the new overflow clip layer then
it's not possible for the new one to be the root is it?

[smcgruer, 2017/02/22 15:26:32]

Done.

+        // ancestor scroller is also the root layer.
+        if (previousOverflowLayer && previousOverflowLayer->isRootLayer()) {
+          layer->layoutObject()
+              ->view()
+              ->frameView()
+              ->removeViewportConstrainedObject(layer->layoutObject());
+        }
+      }
+
+      if (info.lastOverflowClipLayer->isRootLayer()) {
         layer->layoutObject()
             ->view()
             ->frameView()
             ->addViewportConstrainedObject(layer->layoutObject());
-      else if (previousOverflowLayer && previousOverflowLayer->isRootLayer())
-        layer->layoutObject()
-            ->view()
-            ->frameView()
-            ->removeViewportConstrainedObject(layer->layoutObject());
+      }
     }
     layer->layoutObject()->updateStickyPositionConstraints();