gecko-dev/widget/tests/system_font_changes.xul

<?xml version="1.0"?>

<!-- This Source Code Form is subject to the terms of the Mozilla Public
   - License, v. 2.0. If a copy of the MPL was not distributed with this
   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->

<?xml-stylesheet href="chrome://global/skin" type="text/css"?>

<window id="system_font_changes_window"
        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
        width="300"
        height="300"
        onload="start();">

<span id="target" style="font:menu">Hello world</span>

<script type="application/javascript"><![CDATA[
  function is(condition, message) {
    window.opener.wrappedJSObject.SimpleTest.is(condition, message);
  }
  function registerCleanupFunction(func) {
    window.opener.wrappedJSObject.SimpleTest.registerCleanupFunction(func);
  }

  async function waitForFrame() {
    return new Promise(resolve => {
      requestAnimationFrame(resolve);
    });
  }

  async function start() {
    await waitForFrame();

    const originalSystemFont = windowUtils.systemFont;
    registerCleanupFunction(() => {
      windowUtils.systemFont = originalSystemFont;
    });

    windowUtils.systemFont = 'Sans 11';
    is(windowUtils.systemFont, 'Sans 11');

    // Wait for two frames for the safety since the notification for system
    // font changes is asynchronously processed.
    await waitForFrame();
    await waitForFrame();

    const target = document.getElementById('target');
    is(getComputedStyle(target).fontFamily, 'Sans');

    windowUtils.systemFont = 'Serif 11';
    is(windowUtils.systemFont, 'Serif 11');

    await waitForFrame();
    await waitForFrame();

    is(getComputedStyle(target).fontFamily, 'Serif');

    window.close();
    window.opener.wrappedJSObject.SimpleTest.finish();
  }
]]></script>
</window>