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..3340efce14ab633eceb4f12615963c2c46696366 100644
--- a/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.cpp
+++ b/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.cpp
@@ -147,10 +147,23 @@ 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.
+  setNeedsReattachLayoutTree();

[esprehn, 2017/02/02 08:37:10]

This is setting reattach unconditionally on every shadow root which isn't right.
You should never need to force this bit on here.

[nainar, 2017/02/02 23:13:06]

When we call ShadowRoot::recalcStyle() we have no way of knowing when we need to
call ShadowRoot::rebuildLayoutTree() currently only the NeedsReattachLayoutTree
flag is set on the ShadowRoot itself. However it's ancestors dont have this
information. 

I understand that we are indiscriminately calling
ShadowRoot::rebuildLayoutTree() as a result of this. The only other way I see is
to check if the NeedsReattachLayoutTree flag is set and then mark the ancestors
accordingly. Which is the same as calling setNeedsReattachLayoutTree()

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