XCancel Firefox verification workaround

gistfile1.txt

// ==UserScript==
// @name         XCancel Firefox verification workaround
// @namespace    local.xcancel.workaround
// @version      1.0.0
// @description  Work around XCancel's fragile screen/viewport verification check in Firefox.
// @match        https://xcancel.com/*
// @run-at       document-start
// @sandbox      raw
// @grant        none
// @noframes
// ==/UserScript==

(() => {
  "use strict";

  // XCancel only attaches one of its per-request verification headers when
  // screen.width/height exactly match the page's inner viewport. Firefox UI
  // such as the revamped sidebar can make those values differ. Present the
  // viewport dimensions through Screen until the verification page has run,
  // so XCancel's own script supplies the current challenge header and value.
  const prototype = Screen.prototype;
  const originalWidth = Object.getOwnPropertyDescriptor(prototype, "width");
  const originalHeight = Object.getOwnPropertyDescriptor(prototype, "height");
  let installed = false;

  const viewportWidth = () =>
    window.innerWidth || document.documentElement?.clientWidth || 0;
  const viewportHeight = () =>
    window.innerHeight || document.documentElement?.clientHeight || 0;

  const restore = () => {
    if (!installed) return;

    try {
      Object.defineProperty(prototype, "width", originalWidth);
      Object.defineProperty(prototype, "height", originalHeight);
    } finally {
      installed = false;
    }
  };

  try {
    if (!originalWidth?.configurable || !originalHeight?.configurable) {
      throw new Error("Screen dimension properties are not configurable");
    }

    installed = true;
    Object.defineProperty(prototype, "width", {
      configurable: true,
      enumerable: originalWidth.enumerable,
      get: viewportWidth,
    });
    Object.defineProperty(prototype, "height", {
      configurable: true,
      enumerable: originalHeight.enumerable,
      get: viewportHeight,
    });
  } catch (error) {
    restore();
    console.warn("[XCancel workaround] Could not patch screen dimensions:", error);
    return;
  }

  // On a successful reload, restore the native Screen implementation as soon
  // as ordinary XCancel content is identified. Keep the patch active on the
  // verification document because its challenge runs shortly after DOM load.
  const restoreOnContentPage = () => {
    if (!/Verifying your request/i.test(document.title)) {
      restore();
    }
  };

  if (document.readyState === "loading") {
    document.addEventListener("DOMContentLoaded", restoreOnContentPage, {
      once: true,
    });
  } else {
    restoreOnContentPage();
  }
})();