Bug 1443360 - Fix browser_panelUINotifications_fullscreen.js intermittent failure. r=Gijs

MozReview-Commit-ID: FTP3NXqW0oy

--HG--
extra : rebase_source : a9325ed2598c04bc6214ba42b921d4dd23366465

browser/components/customizableui/test/browser_panelUINotifications_fullscreen.js

@@ -19,6 +19,7 @@ add_task(async function testFullscreen() {
   is(doorhanger.id, "appMenu-update-manual-notification", "PanelUI is displaying the update-manual notification.");
 
   let popuphiddenPromise = BrowserTestUtils.waitForEvent(PanelUI.notificationPanel, "popuphidden");
+  document.documentElement.focus();
   EventUtils.synthesizeKey("KEY_F11");
   await popuphiddenPromise;
   await new Promise(executeSoon);