Bug 1677263 - Add a test for select event being async r=masayuki

The asynchronous behavior is introduced by using SetSelectionRange inside Select(). This adds a test for that.

Differential Revision: https://phabricator.services.mozilla.com/D98186

dom/tests/mochitest/chrome/window_focus.xhtml

@@ -433,7 +433,7 @@ function startTest()
   expectFocusShift(() => t19.select(),
                    null, getById("t" + 19), true, "input.select()");
   t19.removeEventListener("select", selectListener, false);
-  ok(selectFired, "select event fires for input");
+  ok(!selectFired, "select event does not fire asynchronously for input");
 
   // mouse clicking
   gLastFocusMethod = fm.FLAG_BYMOUSE;

testing/web-platform/tests/html/semantics/forms/textfieldselection/select-event.html

@@ -92,6 +92,17 @@ els.forEach((el) => {
       // step_wait properly timeouts before the whole test collapses
       await t.step_wait(() => !element.onselect, "event didn't fire", 200, 10);
     }, `${elLabel}: ${action.label} disconnected node`);
+
+    // Intentionally still using promise_test, as assert_unreachable does not
+    // make the test fail inside a listener while t.unreached_func() does.
+    promise_test(async t => {
+      const element = el.cloneNode(true);
+      const listener = t.unreached_func("the select event must not fire synchronously");
+
+      element.onselect = listener;
+      action.action(element);
+      element.onselect = null;
+    }, `${elLabel}: ${action.label} event queue`);
   });
 });
 </script>