Bug 1589102 - Part 12: Fix file_scrollRestoration.html to wait for about:blank to load before proceeding, r=peterv

Differential Revision: https://phabricator.services.mozilla.com/D87933

docshell/test/navigation/file_scrollRestoration.html

@@ -39,7 +39,7 @@
             opener.is(history.scrollRestoration, "manual", "Should have the same scrollRestoration as before reload.");
             document.getElementById("bottom").scrollIntoView();
             window.onunload = null; // Should get bfcache behavior.
-            opener.setTimeout("SpecialPowers.wrap(testWindow).history.back();", 250);
+            window.opener.case3WaitForBlank();
             window.location.href = "about:blank";
             break;
           }

docshell/test/navigation/test_sessionhistory.html

@@ -54,6 +54,14 @@ function nextTest_() {
   }
 }
 
+async function case3WaitForBlank() {
+  let loaded = SimpleTest.promiseWaitForCondition(() => {
+    return testWindow.location.href == "about:blank";
+  });
+  await loaded;
+  SpecialPowers.wrap(testWindow).history.back();
+}
+
 function nextTest() {
   setTimeout(nextTest_, 0);
 }