alwaysonfocus.user.js

// ==UserScript==
// @name          Always on focus
// @namespace     https://minhquang.xyz/focustest
// @author        moi, daijro
// @version       1.0.0
// @description   Ngăn các trang web biết rằng bạn đã chuyển tab hoặc không ở trong trang web.
// @include       *
// @run-at        document-start
// ==/UserScript==

onblur = null;
blurred = false;
document.hasFocus = function () {return true;};
window.onFocus = function () {return true;};
Object.defineProperty(document, "hidden", { value : false});
Object.defineProperty(document, "mozHidden", { value : false});
Object.defineProperty(document, "msHidden", { value : false});
Object.defineProperty(document, "webkitHidden", { value : false});
document.onvisibilitychange = undefined;
for (const event_name of ["visibilitychange", "webkitvisibilitychange", "blur", "mozvisibilitychange", "msvisibilitychange"]) 
    {
    window.addEventListener(event_name, function (event) {
        if (event.type === 'blur' && event.target instanceof HTMLInputElement) {
            return;
        }
        event.stopImmediatePropagation();
    }, true);
}
console.log("%cDISABLED!", "color: red; font-size: x-large");
//CODE BY MINHDEPTRAI