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..0bd4f35f2db6db7e4ae725e4830125b9b3d6b8b6 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp
@@ -102,24 +102,30 @@ 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 ancestor scroller has changed and the previous one was the
+        // root layer, we are no longer viewport constrained.
+        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();