Call Element::rebuildLayoutTree from Document::updateStyle directly

third_party/WebKit/Source/core/dom/shadow/ShadowRoot.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Neither the name of Google Inc. nor the names of its
@@ skipping 111 matching lines @@
   // ShadowRoot doesn't support custom callbacks.
   DCHECK(!hasCustomStyleCallbacks());
 
   StyleSharingDepthScope sharingScope(*this);
 
   if (getStyleChangeType() >= SubtreeStyleChange)
     change = Force;
 
   // There's no style to update so just calling recalcStyle means we're updated.
   clearNeedsStyleRecalc();
-  clearNeedsReattachLayoutTree();
 
   recalcDescendantStyles(change);
   clearChildNeedsStyleRecalc();
+  setNeedsReattachLayoutTree();

[esprehn, 2016/12/14 04:30:48]

Hmm wait this doesn't seem right, why are you forcing a reattach of the
ShadowRoot on every recalcStyle?

[nainar, 2016/12/14 22:21:54]

So in the following test case:

<script>
onload = function() {
   var sr = document.getElementById('host').createShadowRoot();
}
</script>
</div>
<div id="host">

When we create the ShadowRoot we set NeedsReattachLayoutTree flag on it. It's
parent div host never has its childNeedsReattachLayoutTree flag set. And hence
it doesnt enter the childNeedsReattachLayoutTree part of
Element::rebuildLayoutTree and never clears the flag on the ShadowRoot.

+}
+
+void ShadowRoot::rebuildLayoutTree() {
+  DCHECK(!needsStyleRecalc());
+  // ShadowRoot doesn't support custom callbacks.
+  DCHECK(!hasCustomStyleCallbacks());
+
+  clearNeedsReattachLayoutTree();
+  rebuildChildrenLayoutTrees();
   clearChildNeedsReattachLayoutTree();

[esprehn, 2016/12/14 04:30:48]

This bit is cleared inside rebuildChildrenLayoutTrees? I don't think you need to
clear it here.

[nainar, 2016/12/14 22:21:54]

ShadowRoot::rebuildLayoutTree() also gets called by
Element::rebuildShadowRootLayoutTree() which doesn't clear the bit. 

 }
 
 void ShadowRoot::attachLayoutTree(const AttachContext& context) {
   StyleSharingDepthScope sharingScope(*this);
   DocumentFragment::attachLayoutTree(context);
 }
 
 void ShadowRoot::detachLayoutTree(const AttachContext& context) {
   if (context.clearInvalidation)
     document().styleEngine().styleInvalidator().clearInvalidation(*this);
@@ skipping 169 matching lines @@
       ostream << "ShadowRootType::Open";
       break;
     case ShadowRootType::Closed:
       ostream << "ShadowRootType::Closed";
       break;
   }
   return ostream;
 }
 
 }  // namespace blink