Call Element::rebuildLayoutTree from Document::updateStyle directly

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

Index: third_party/WebKit/Source/core/dom/shadow/ShadowRoot.cpp
diff --git a/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.cpp b/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.cpp
index fad2a8a0642d040e088ab104eaa8a8e0aeb3a67a..cbb2a96377c5f441800a611749e842eccb468bbe 100644
--- a/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.cpp
+++ b/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.cpp
@@ -129,10 +129,19 @@ void ShadowRoot::recalcStyle(StyleRecalcChange change) {
 
   // 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. 

 }