Bug 1488871 - Add an explicit style flush to make sure the added textbox and popupset is styled before proceeding tests in browser_UrlbarInput_unit.js. r=adw

Differential Revision: https://phabricator.services.mozilla.com/D22641

--HG--
extra : moz-landing-system : lando

browser/components/urlbar/tests/browser/browser_UrlbarInput_unit.js

@@ -93,6 +93,10 @@ async function withNewWindow(callback) {
 
   let input = new UrlbarInput(inputOptions);
 
+  // Flush pending styles explicitely to make sure the added textbox and
+  // popupset are styled before proceeding test.  See bug 1488871 comment 26.
+  doc.documentElement.getBoundingClientRect();
+
   await callback(input);
 
   await BrowserTestUtils.closeWindow(win);