Mark elements as viewport constrained when they become sticky positioned

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/compositing/CompositingInputsUpdater.h"
 
 #include "core/dom/Document.h"
 #include "core/frame/FrameHost.h"
 #include "core/frame/FrameView.h"
 #include "core/layout/LayoutBlock.h"
@@ skipping 84 matching lines @@
 void CompositingInputsUpdater::updateRecursive(PaintLayer* layer,
                                                UpdateType updateType,
                                                AncestorInfo info) {
   if (!layer->childNeedsCompositingInputsUpdate() && updateType != ForceUpdate)
     return;
 
   const PaintLayer* previousOverflowLayer = layer->ancestorOverflowLayer();
   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 (!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();
 
     // Sticky position constraints and ancestor overflow scroller affect
     // the sticky layer position, so we need to update it again here.
     // TODO(flackr): This should be refactored in the future to be clearer
     // (i.e. update layer position and ancestor inputs updates in the
     // same walk)
     layer->updateLayerPosition();
@@ skipping 136 matching lines @@
   ASSERT(!layer->needsCompositingInputsUpdate());
 
   for (PaintLayer* child = layer->firstChild(); child;
        child = child->nextSibling())
     assertNeedsCompositingInputsUpdateBitsCleared(child);
 }
 
 #endif
 
 }  // namespace blink