Force fullscreen UI on sites like YouTube without actually making your browser window fullscreen

fake-fullscreen.js

let fullscreenElement;
Object.defineProperty(document, "fullscreenElement", {
  get() {
    return fullscreenElement;
  },
  set(value) {
    fullscreenElement = value;
  },
});

Element.prototype.requestFullscreen = async function () {
  document.fullscreenElement = this;
  document.dispatchEvent(new Event("fullscreenchange"));
};

document.exitFullscreen = () => {
  document.fullscreenElement = null;
  document.dispatchEvent(new Event("fullscreenchange"));
};