Bug 1648435 - Make dom/events/test/test_mouse_enterleave_iframe.html waiting apz stable before starting tests; r=smaug

This test uses synthesizeNativeMouseMove to synthesize mouse events, the synthesized
native events would go through apz, so wait apz getting stable before starting test
to avoid possible timing problem that causes intermittent failure.

Differential Revision: https://phabricator.services.mozilla.com/D103192

dom/events/test/test_mouse_enterleave_iframe.html

@@ -3,6 +3,8 @@
 <title>Test mouseenter and mouseleave for iframe.</title>
 <script src="/tests/SimpleTest/SimpleTest.js"></script>
 <script src="/tests/SimpleTest/EventUtils.js"></script>
+<script src="/tests/SimpleTest/paint_listener.js"></script>
+<script src="/tests/gfx/layers/apz/test/mochitest/apz_test_utils.js"></script>
 <link rel="stylesheet" href="/tests/SimpleTest/test.css"/>
 <style>
 #start {
@@ -157,6 +159,9 @@ add_task(async function init() {
   // Wait for focus before starting tests.
   await SimpleTest.promiseFocus();
 
+  // Wait for apz getting stable.
+  await waitUntilApzStable();
+
   // Move mouse to initial position.
   await moveMouseToInitialPosition();
 });
@@ -262,6 +267,8 @@ add_task(async function testMouseEnterLeaveSwitchWindow() {
   win.close();
   // Trigger a reflow which will generate synthesized mouse move event.
   reflow();
+  // Wait for apz getting stable.
+  await waitUntilApzStable();
   synthesizeNativeMouseMove(target, targetRect.width / 2, targetRect.height / 2);
   await promise;