--- a/browser/components/customizableui/content/panelUI.js
+++ b/browser/components/customizableui/content/panelUI.js
@@ -324,16 +324,17 @@ const PanelUI = {
     } else if (!aAnchor.open) {
       aAnchor.open = true;
       // Emit the ViewShowing event so that the widget definition has a chance
       // to lazily populate the subview with things.
       let evt = document.createEvent("CustomEvent");
       evt.initCustomEvent("ViewShowing", true, true, viewNode);
       viewNode.dispatchEvent(evt);
       if (evt.defaultPrevented) {
+        aAnchor.open = false;
         return;
       }
 
       let tempPanel = document.createElement("panel");
       tempPanel.setAttribute("type", "arrow");
       tempPanel.setAttribute("id", "customizationui-widget-panel");
       tempPanel.setAttribute("class", "cui-widget-panel");
       tempPanel.setAttribute("viewId", aViewId);