Bug 1607984 - P15. Wait for the load to start before setting the event handler. r=Jamie

addA11yLoadEvent gets the contentWindow and wait for the document from that window to fire the load event.

Enabling the DocumentChannel for parent process load (or here in non-e10s mode) we have one extra event loop before the load starts.
So the window passed to addAllyLoadEvent would have been of the about:blank page.

The current code was based on an observable behaviour which was that the load was occuring synchronously.

DocumentChannel broke that assumption.

Differential Revision: https://phabricator.services.mozilla.com/D70799

accessible/tests/mochitest/browser.js

@@ -135,6 +135,11 @@ function openBrowserWindowIntl() {
 
 function startBrowserTests() {
   if (gBrowserContext.startURL) {
+    // Make sure the window is the one loading our URL.
+    if (currentBrowser().contentWindow.location != gBrowserContext.startURL) {
+      setTimeout(startBrowserTests, 0);
+      return;
+    }
     // wait for load
     addA11yLoadEvent(gBrowserContext.testFunc, currentBrowser().contentWindow);
   } else {