Bug 1827856 - Speed up GetClosestAnonymousSubtreeRoot for UA widgets. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D175353

dom/base/ShadowRoot.cpp

@@ -61,6 +61,11 @@ ShadowRoot::ShadowRoot(Element* aElement, ShadowRootMode aMode,
       mSlotAssignment(aSlotAssignment),
       mIsDetailsShadowTree(aElement->IsHTMLElement(nsGkAtoms::details)),
       mIsAvailableToElementInternals(false) {
+  // nsINode.h relies on this.
+  MOZ_ASSERT(static_cast<nsINode*>(this) == reinterpret_cast<nsINode*>(this));
+  MOZ_ASSERT(static_cast<nsIContent*>(this) ==
+             reinterpret_cast<nsIContent*>(this));
+
   SetHost(aElement);
 
   // Nodes in a shadow tree should never store a value

dom/base/nsINode.h

@@ -1410,9 +1410,15 @@ class nsINode : public mozilla::dom::EventTarget {
    */
   nsIContent* GetClosestNativeAnonymousSubtreeRoot() const {
     if (!IsInNativeAnonymousSubtree()) {
+      MOZ_ASSERT(!HasBeenInUAWidget(), "UA widget implies anonymous");
       return nullptr;
     }
     MOZ_ASSERT(IsContent(), "How did non-content end up in NAC?");
+    if (HasBeenInUAWidget()) {
+      // reinterpret_cast because in this header we don't know ShadowRoot is an
+      // nsIContent. ShadowRoot constructor asserts this is correct.
+      return reinterpret_cast<nsIContent*>(GetContainingShadow());
+    }
     for (const nsINode* node = this; node; node = node->GetParentNode()) {
       if (node->IsRootOfNativeAnonymousSubtree()) {
         return const_cast<nsINode*>(node)->AsContent();