Call Element::rebuildLayoutTree from Document::updateStyle directly

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

 /*
  * Copyright (C) 2012 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 64 matching lines @@
     m_elementShadowV0 = ElementShadowV0::create(*this);
   }
 
   ShadowRoot* shadowRoot = ShadowRoot::create(shadowHost.document(), type);
   shadowRoot->setParentOrShadowHostNode(&shadowHost);
   shadowRoot->setParentTreeScope(shadowHost.treeScope());
   appendShadowRoot(*shadowRoot);
   setNeedsDistributionRecalc();
 
   shadowRoot->insertedInto(&shadowHost);
-  shadowHost.setChildNeedsStyleRecalc();

[esprehn, 2017/02/16 21:24:33]

I think we probably want to leave this, while technically it's not needed, it
preserves the invariant that if the shaodwRoot needs a recalc (which it does
since it's brand new), that the host has the child bit set.

(I know this was my suggestion.)

[nainar, 2017/02/16 22:26:09]

Sure :)

   shadowHost.setNeedsStyleRecalc(
       SubtreeStyleChange,
       StyleChangeReasonForTracing::create(StyleChangeReason::Shadow));
+  shadowRoot->setNeedsReattachLayoutTree();
 
   InspectorInstrumentation::didPushShadowRoot(&shadowHost, shadowRoot);
 
   return *shadowRoot;
 }
 
 void ElementShadow::appendShadowRoot(ShadowRoot& shadowRoot) {
   if (!m_shadowRoot) {
     m_shadowRoot = &shadowRoot;
     return;
@@ skipping 64 matching lines @@
   visitor->trace(m_elementShadowV0);
   visitor->trace(m_shadowRoot);
 }
 
 DEFINE_TRACE_WRAPPERS(ElementShadow) {
   visitor->traceWrappers(m_elementShadowV0);
   visitor->traceWrappers(m_shadowRoot);
 }
 
 }  // namespace blink