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 c72f2a0cdc84b8557a6fa137266e55be6b0c5142..621e4f002e16c87ce2e7593b3238f7e5d66f73de 100644
--- a/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.cpp
+++ b/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.cpp
@@ -147,10 +147,24 @@ void ShadowRoot::recalcStyle(StyleRecalcChange change) {
 
   // There's no style to update so just calling recalcStyle means we're updated.
   clearNeedsStyleRecalc();
-  clearNeedsReattachLayoutTree();
 
   recalcDescendantStyles(change);
   clearChildNeedsStyleRecalc();
+  // Currently only the NeedsReattachLayoutTree flag is set on the ShadowRoot.
+  // Need to tell the ancestor nodes that this one needs reattach as well
+  // otherwise we will never rebuild the LayoutTree.

[esprehn, 2017/02/02 23:31:03]

I see what you're missing is a call to host->setChildNeedsReattachLayoutTree()
inside the shadow root creation path. Look at the code inside ElementShadow
addShadowRoot. Youll see it already sets bits like this. You just need to do the
same. Note you probably need code for both v1 and v0 shadow roots.

+  if (needsReattachLayoutTree())
+    markAncestorsWithChildNeedsReattachLayoutTree();
+}
+
+void ShadowRoot::rebuildLayoutTree() {
+  // ShadowRoot doesn't support custom callbacks.
+  DCHECK(!hasCustomStyleCallbacks());
+
+  StyleSharingDepthScope sharingScope(*this);
+
+  clearNeedsReattachLayoutTree();
+  rebuildChildrenLayoutTrees();
   clearChildNeedsReattachLayoutTree();
 }