Bug 1488953 - Wait for a scrollend event to make sure the scrolling has finished before proceeding the test. r=bgrins

Differential Revision: https://phabricator.services.mozilla.com/D33614

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

browser/base/content/test/tabs/browser_tabCloseSpacer.js

@@ -48,7 +48,11 @@ async function overflowTabs() {
   while (gBrowser.tabs.length < tabCountForOverflow) {
     BrowserTestUtils.addTab(gBrowser, "about:blank", { skipAnimation: true, index: 0 });
   }
-  await window.promiseDocumentFlushed(() => {});
+
+  // Make sure scrolling finished.
+  await new Promise(resolve => {
+    arrowScrollbox.addEventListener("scrollend", resolve, { once: true });
+  });
 }
 
 function getLastCloseButton() {