 that the navigation doesn't get converted
  // into a replace navigation.
  await ensureWindowLoadEventFired(t);
  await navigation.navigate("#").finished;
  await i.contentWindow.navigation.navigate("#").finished;
  assert_equals(navigation.entries().length, start_length + 1);
  assert_equals(i.contentWindow.navigation.entries().length, 2);
  assert_equals(navigation.currentEntry.index, start_index + 1);
  assert_equals(i.contentWindow.navigation.currentEntry.index, 1);

  navigation.onnavigate = e => e.p