bug 864247. If PermitUnload is called while another PermitUnload is being processed, simply allow the unload to happen. r=roc

layout/base/nsDocumentViewer.cpp

@@ -428,6 +428,7 @@ protected:
   //   may consider splitting these out into a subclass
   unsigned      mIsSticky : 1;
   unsigned      mInPermitUnload : 1;
+  unsigned      mInPermitUnloadPrompt: 1;
 
 #ifdef NS_PRINTING
   unsigned      mClosingWhilePrinting : 1;
@@ -1090,7 +1091,10 @@ nsDocumentViewer::PermitUnload(bool aCallerClosesWindow, bool *aPermitUnload)
 {
   *aPermitUnload = true;
 
-  if (!mDocument || mInPermitUnload || mCallerIsClosingWindow) {
+  if (!mDocument
+   || mInPermitUnload
+   || mCallerIsClosingWindow
+   || mInPermitUnloadPrompt) {
     return NS_OK;
   }
 
@@ -1183,9 +1187,11 @@ nsDocumentViewer::PermitUnload(bool aCallerClosesWindow, bool *aPermitUnload)
                              (nsIPrompt::BUTTON_TITLE_IS_STRING * nsIPrompt::BUTTON_POS_1));
 
       nsAutoSyncOperation sync(mDocument);
+      mInPermitUnloadPrompt = true;
       rv = prompt->ConfirmEx(title, message, buttonFlags,
                              leaveLabel, stayLabel, nullptr, nullptr,
                              &dummy, &buttonPressed);
+      mInPermitUnloadPrompt = false;
       NS_ENSURE_SUCCESS(rv, rv);
 
       // Button 0 == leave, button 1 == stay