#388: M1354564

browser/base/content/tabbrowser.xml

@@ -1008,6 +1008,8 @@
             var newBrowser = this.getBrowserAtIndex(this.tabContainer.selectedIndex);
             if (this.mCurrentBrowser == newBrowser && !aForceUpdate)
               return;
+            if (!aForceUpdate)
+              document.commandDispatcher.lock();
 
 /*
             if (!aForceUpdate) {
@@ -1204,6 +1206,8 @@
             this.tabContainer._setPositionalAttributes();
 
             //if (!gMultiProcessBrowser) {
+              document.commandDispatcher.unlock();
+
               let event = new CustomEvent("TabSwitchDone", {
                 bubbles: true,
                 cancelable: true
@@ -3203,6 +3207,8 @@ let remote = false;
                 fromBrowser.setAttribute("type", "content-targetable");
               }
 
+              document.commandDispatcher.unlock();
+
               let event = new CustomEvent("TabSwitchDone", {
                 bubbles: true,
                 cancelable: true

dom/interfaces/xul/nsIDOMXULCommandDispatcher.idl

@@ -31,4 +31,9 @@ interface nsIDOMXULCommandDispatcher : nsISupports
   void rewindFocus();
   void advanceFocusIntoSubtree(in nsIDOMElement elt);
   attribute boolean suppressFocusScroll;
+
+  // When locked, command updating is batched until unlocked. Always ensure that
+  // lock and unlock is called in a pair.
+  void lock();
+  void unlock();
 };

dom/xul/nsXULCommandDispatcher.cpp

@@ -42,7 +42,7 @@ static LazyLogModule gCommandLog("nsXULCommandDispatcher");
 ////////////////////////////////////////////////////////////////////////
 
 nsXULCommandDispatcher::nsXULCommandDispatcher(nsIDocument* aDocument)
-    : mDocument(aDocument), mUpdaters(nullptr)
+    : mDocument(aDocument), mUpdaters(nullptr), mLocked(false)
 {
 }
 
@@ -351,6 +351,14 @@ nsXULCommandDispatcher::RemoveCommandUpdater(nsIDOMElement* aElement)
 NS_IMETHODIMP
 nsXULCommandDispatcher::UpdateCommands(const nsAString& aEventName)
 {
+  if (mLocked) {
+    if (!mPendingUpdates.Contains(aEventName)) {
+      mPendingUpdates.AppendElement(aEventName);
+    }
+
+    return NS_OK;
+  }
+
   nsAutoString id;
   nsCOMPtr<nsIDOMElement> element;
   GetFocusedElement(getter_AddRefs(element));
@@ -458,3 +466,30 @@ nsXULCommandDispatcher::SetSuppressFocusScroll(bool aSuppressFocusScroll)
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsXULCommandDispatcher::Lock()
+{
+  // Since locking is used only as a performance optimization, we don't worry
+  // about nested lock calls. If that does happen, it just means we will unlock
+  // and process updates earlier.
+  mLocked = true;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsXULCommandDispatcher::Unlock()
+{
+  if (mLocked) {
+    mLocked = false;
+
+    // Handle any pending updates one at a time. In the unlikely case where a
+    // lock is added during the update, break out.
+    while (!mLocked && mPendingUpdates.Length() > 0) {
+      nsString name = mPendingUpdates.ElementAt(0);
+      mPendingUpdates.RemoveElementAt(0);
+      UpdateCommands(name);
+    }
+  }
+
+  return NS_OK;
+}

dom/xul/nsXULCommandDispatcher.h

@@ -68,6 +68,9 @@ protected:
 
     bool Matches(const nsString& aList, 
                    const nsAString& aElement);
+
+    bool mLocked;
+    nsTArray<nsString> mPendingUpdates;
 };
 
 #endif // nsXULCommandDispatcher_h__