Bug 1643998: Ignore load events from stale actors in browserLoaded helper. r=nika

Differential Revision: https://phabricator.services.mozilla.com/D117570

browser/base/content/test/tabs/browser.ini

@@ -81,7 +81,6 @@ support-files = file_new_tab_page.html
 skip-if =
   !e10s
   os == 'linux' && debug && bits == 64  # Pref and test only relevant for e10s, Bug 1581500.
-  fission && (os == "mac" || os == 'linux')  # Bug 1713893 - new Fission platform triage
 [browser_privilegedmozilla_process_pref.js]
 skip-if = !e10s # Pref and test only relevant for e10s.
 [browser_newwindow_tabstrip_overflow.js]

testing/mochitest/BrowserTestUtils/BrowserTestUtilsParent.jsm

@@ -11,6 +11,15 @@ class BrowserTestUtilsParent extends JSWindowActorParent {
     switch (aMessage.name) {
       case "DOMContentLoaded":
       case "load": {
+        // Don't dispatch events that came from stale actors.
+        let bc = this.browsingContext;
+        if (
+          (bc.embedderElement && bc.embedderElement.browsingContext != bc) ||
+          !(this.manager && this.manager.isCurrentGlobal)
+        ) {
+          return;
+        }
+
         let event = new CustomEvent(
           `BrowserTestUtils:ContentEvent:${aMessage.name}`,
           {