test_textScaleFactor_system_font.html

Enable keyboard shortcuts

This test gets skipped with pattern: display == 'wayland' && os_version == '22.04'
Manifest: widget/tests/mochitest.toml

<!DOCTYPE HTML>

<html>

<head>

  <meta charset="utf-8">

  <title>Test that system font sizing is independent from ui.textScaleFactor</title>

  <script src="/tests/SimpleTest/SimpleTest.js"></script>

  <link rel="stylesheet" href="/tests/SimpleTest/test.css"/>

  <style>

    p { width: max-content }

    #menu { font: menu }

  </style>

</head>

<body>

  <p id="menu">"menu" text.</p>

  <p id="default">Default text.</p>

</body>

<script>

"use strict";

const { AppConstants } = SpecialPowers.ChromeUtils.importESModule(

  "resource://gre/modules/AppConstants.sys.mjs"

);

// Returns a Number for the font size in CSS pixels.

function elementFontSize(element) {

  return parseFloat(getComputedStyle(element).getPropertyValue("font-size"));

// "look-and-feel-changed" may be dispatched twice: once for the pref

// change and once after receiving the new values from

// ContentChild::RecvThemeChanged().

// pushPrefEnv() resolves after the former. This resolves after the latter.

function promiseNewFontSizeOnThemeChange(element) {

  return new Promise(resolve => {

    const lastSize = elementFontSize(element);

    function ThemeChanged() {

      const size = elementFontSize(element);

      if (size != lastSize) {

        resolve(size);

    // "look-and-feel-changed" is dispatched before the style system is flushed,

    // https://searchfox.org/mozilla-central/rev/380fc5571b039fd453b45bbb64ed13146fe9b066/layout/base/nsPresContext.cpp#1684,1703-1705

    // so use an async observer to get a notification after style changes.

    SpecialPowers.addAsyncObserver(ThemeChanged, "look-and-feel-changed");

    SimpleTest.registerCleanupFunction(function() {

      SpecialPowers.removeAsyncObserver(ThemeChanged, "look-and-feel-changed");

});

});

function fuzzyCompareLength(actual, expected, message, tolerance, expectFn) {

  expectFn(Math.abs(actual-expected) <= tolerance,

           `${message} - got ${actual}, expected ${expected} +/- ${tolerance}`);

add_task(async () => {

  // MOZ_HEADLESS is set in content processes with GTK regardless of the

  // headless state of the parent process.  Check the parent state.

  const headless = await SpecialPowers.spawnChrome([], function get_headless() {

    return Services.env.get("MOZ_HEADLESS");

});

  // LookAndFeel::TextScaleFactor::FloatID is implemented only for WINNT and

  // GTK.  ui.textScaleFactor happens to scale CSS pixels on other platforms

  // but system font integration has not been implemented.

  const expectSuccess = AppConstants.MOZ_WIDGET_TOOLKIT == "windows" ||

         (AppConstants.MOZ_WIDGET_TOOLKIT == "gtk" &&

          // Headless GTK doesn't get system font sizes from the system, but

          // uses sizes fixed in CSS pixels.

          !headless);

  async function setScaleAndPromiseFontSize(scale, element) {

    const prefPromise = SpecialPowers.pushPrefEnv({

      set: [["ui.textScaleFactor", scale]],

});

    if (!expectSuccess) {

      // The size is not expected to change but get it afresh to check our

      // assumption.

      await prefPromise;

      return elementFontSize(element);

    const [size] = await Promise.all([

      promiseNewFontSizeOnThemeChange(element),

      prefPromise,

]);

    return size;

  const menu = document.getElementById("menu");

  const def = document.getElementById("default");

  // Choose a scaleFactor value different enough from possible default values

  // that app unit rounding does not prevent a change in devicePixelRatio.

  // A scaleFactor of 120 also has no rounding of app units per dev pixel.

  const referenceScale = 120;

  const menuSize1 = await setScaleAndPromiseFontSize(referenceScale, menu);

  const menuRect1 = menu.getBoundingClientRect();

  const defSize1 = elementFontSize(def);

  const defRect1 = def.getBoundingClientRect();

  const expectFn = expectSuccess ? ok : todo;

  const menuSize2 = await setScaleAndPromiseFontSize(2*referenceScale, menu);

    const singlePrecisionULP = Math.pow(2, -23);

    // Precision seems to be lost in the conversion to decimal string for the

    // property value.

    const reltolerance = 30 * singlePrecisionULP;

    fuzzyCompareLength(menuSize2, menuSize1/2, "size of menu font",

                       reltolerance*menuSize1/2, expectFn);

    const menuRect2 = menu.getBoundingClientRect();

    // The menu font text renders exactly the same and app-unit rects are

    // equal, but the DOMRect conversion is rounded to 1/65536 CSS pixels.

    // https://searchfox.org/mozilla-central/rev/380fc5571b039fd453b45bbb64ed13146fe9b066/dom/base/DOMRect.cpp#151-159

    // See also https://bugzilla.mozilla.org/show_bug.cgi?id=1640441#c28

    const absTolerance = 1/65536

    fuzzyCompareLength(menuRect2.width, menuRect1.width/2,

                       "width of menu font <p> in px", absTolerance, expectFn);

    fuzzyCompareLength(menuRect2.height, menuRect1.height/2,

                       "height of menu font <p> in px", absTolerance, expectFn);

  const defSize2 = elementFontSize(def);

  is(defSize2, defSize1, "size of default font");

    const defRect2 = def.getBoundingClientRect();

    // Wider tolerance for hinting and snapping

    const relTolerance = 1/12;

    fuzzyCompareLength(defRect2.width, defRect1.width,

                       "width of default font <p> in px",

                       relTolerance*defRect1.width, ok);

    fuzzyCompareLength(defRect2.height, defRect1.height,

                       "height of default font <p> in px",

                       relTolerance*defRect1.height, ok);

});

</script>

</html>