Handle nested position:sticky elements

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 1910 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;
     PaintLayer* layer = toLayoutBoxModelObject(layoutObject)->layer();
-    if (layoutObject->style()->position() == StickyPosition) {
+    StickyConstraintsMap constraintsMap =
+        layer->getScrollableArea()->stickyConstraintsMap();
+    if (layoutObject->style()->position() == StickyPosition &&
+        constraintsMap.contains(layer) &&

[flackr, 2017/01/20 02:42:44]

Do you know if we do actualy sometimes end up with missing / dirty constraints
at this time? It seems like after scroll would be a clean part of the lifecycle.

[smcgruer, 2017/01/20 17:18:14]

It appears so. I don't know the exact reason, but adding:

CHECK(layoutObject->style()->position() != StickyPosition || 
constraintsMap.contains(layer));

Causes a CHECK the second you scroll http://output.jsbin.com/lukekot

I can ping you the stack-trace off bug if you want to take a closer look.

+        !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 3052 matching lines @@
   // This is the top-level frame, so no mapping necessary.
   if (m_frame->isMainFrame())
     return;
 
   LayoutRect viewportIntersectionRect(remoteViewportIntersection());
   transformState.move(
       LayoutSize(-viewportIntersectionRect.x(), -viewportIntersectionRect.y()));
 }
 
 }  // namespace blink