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 (info.lastOverflowClipLayer->isRootLayer())
+        // 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();
 
     // 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 135 matching lines @@
   ASSERT(!layer->needsCompositingInputsUpdate());
 
   for (PaintLayer* child = layer->firstChild(); child;
        child = child->nextSibling())
     assertNeedsCompositingInputsUpdateBitsCleared(child);
 }
 
 #endif
 
 }  // namespace blink