Handle nested position:sticky elements correctly (main thread)

third_party/WebKit/Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 1939 matching lines @@
 void FrameView::didScrollTimerFired(TimerBase*) {
   if (m_frame->document() && !m_frame->document()->layoutViewItem().isNull())
     m_frame->document()->fetcher()->updateAllImageResourcePriorities();
 }
 
 void FrameView::updateLayersAndCompositingAfterScrollIfNeeded() {
   // Nothing to do after scrolling if there are no fixed position elements.
   if (!hasViewportConstrainedObjects())
     return;
 
-  // Update sticky position objects which are stuck to the viewport.
+  // Update sticky position objects which are stuck to the viewport. In order to
+  // correctly compute the sticky position offsets the layers must be visited
+  // top-down, so start at the 'root' sticky elements and recurse downwards.
   for (const auto& viewportConstrainedObject : *m_viewportConstrainedObjects) {
     LayoutObject* layoutObject = viewportConstrainedObject;
+    if (layoutObject->style()->position() != EPosition::kSticky)
+      continue;
+
     PaintLayer* layer = toLayoutBoxModelObject(layoutObject)->layer();
-    if (layoutObject->style()->position() == EPosition::kSticky) {
+    StickyConstraintsMap constraintsMap = layer->ancestorOverflowLayer()
+                                              ->getScrollableArea()
+                                              ->stickyConstraintsMap();
+    if (constraintsMap.contains(layer) &&
+        !constraintsMap.get(layer).hasAncestorStickyElement()) {
       // TODO(skobes): Resolve circular dependency between scroll offset and
       // compositing state, and remove this disabler. https://crbug.com/420741
       DisableCompositingQueryAsserts disabler;
       layer->updateLayerPositionsAfterOverflowScroll();
       layoutObject->setMayNeedPaintInvalidationSubtree();
     }
   }
 
   // If there fixed position elements, scrolling may cause compositing layers to
   // change.  Update widget and layer positions after scrolling, but only if
@@ skipping 3210 matching lines @@
 void FrameView::setAnimationHost(
     std::unique_ptr<CompositorAnimationHost> host) {
   m_animationHost = std::move(host);
 }
 
 LayoutUnit FrameView::caretWidth() const {
   return LayoutUnit(getHostWindow()->windowToViewportScalar(1));
 }
 
 }  // namespace blink