Bug 1437811 - Part 2 - Prepare the main view just before opening the panel. r=Gijs

This allows the ViewShowing event for the main view to prevent the panel from opening. It also avoids setting up the main view if the panel is not opened.

MozReview-Commit-ID: LK8tBcz6lkK

browser/components/customizableui/content/panelUI.js

@@ -422,10 +422,6 @@ const PanelUI = {
         await oldMultiView.showMainView();
       }
 
-      let viewShown = false;
-      let listener = () => viewShown = true;
-      viewNode.addEventListener("ViewShown", listener, {once: true});
-
       let multiView = document.createElement("panelmultiview");
       multiView.setAttribute("id", "customizationui-widget-multiview");
       multiView.setAttribute("viewCacheId", "appMenu-viewCache");
@@ -435,6 +431,7 @@ const PanelUI = {
       tempPanel.appendChild(multiView);
       viewNode.classList.add("cui-widget-panelview");
 
+      let viewShown = false;
       let panelRemover = () => {
         viewNode.classList.remove("cui-widget-panelview");
         if (viewShown) {
@@ -449,18 +446,6 @@ const PanelUI = {
         tempPanel.remove();
       };
 
-      // Wait until all the tasks needed to show a view are done.
-      await multiView.currentShowPromise;
-
-      if (!viewShown) {
-        viewNode.removeEventListener("ViewShown", listener);
-        panelRemover();
-        return;
-      }
-
-      CustomizableUI.addPanelCloseListeners(tempPanel);
-      tempPanel.addEventListener("popuphidden", panelRemover);
-
       if (aAnchor.parentNode.id == "PersonalToolbar") {
         tempPanel.classList.add("bookmarks-toolbar");
       }
@@ -471,10 +456,21 @@ const PanelUI = {
         anchor.setAttribute("consumeanchor", aAnchor.id);
       }
 
-      PanelMultiView.openPopup(tempPanel, anchor, {
-        position: "bottomcenter topright",
-        triggerEvent: domEvent,
-      }).catch(Cu.reportError);
+      try {
+        viewShown = await PanelMultiView.openPopup(tempPanel, anchor, {
+          position: "bottomcenter topright",
+          triggerEvent: domEvent,
+        });
+      } catch (ex) {
+        Cu.reportError(ex);
+      }
+
+      if (viewShown) {
+        CustomizableUI.addPanelCloseListeners(tempPanel);
+        tempPanel.addEventListener("popuphidden", panelRemover);
+      } else {
+        panelRemover();
+      }
     }
   },