Handle nested position:sticky elements

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

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 43718b3c8601adccf367e8306bbb5b33f57556d1..9ce58d6376a6e8de78c1da81dc16fd5f3bb1e273 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -1928,11 +1928,17 @@ void FrameView::updateLayersAndCompositingAfterScrollIfNeeded() {
   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;