Bug 1217129: Part 2 - Clear open state of view widgets if view opening is prevented. r=gijs

--HG--
extra : commitid : FFUeRcZgH9L
extra : rebase_source : bb1cdbc7a0f37bb13205df31e1fbf689488f5ffd

browser/components/customizableui/content/panelUI.js

@@ -329,6 +329,7 @@ const PanelUI = {
       evt.initCustomEvent("ViewShowing", true, true, viewNode);
       viewNode.dispatchEvent(evt);
       if (evt.defaultPrevented) {
+        aAnchor.open = false;
         return;
       }