Clear disablePictureInPicture attributes from videos.
// ==UserScript==
// @name         Remove PiP Disable Attributes  
// @version      1.0
// @description  Clear disablePictureInPicture attributes from videos.
// @author       Polywock
// @include      http://*
// @include      https://*
// @namespace    https://github.com/polywock
// ==/UserScript==
window.addEventListener("play", function(e) {
    e.target.removeAttribute("disablePictureInPicture")
}, true)
window.addEventListener("pause", function(e) {
    e.target.removeAttribute("disablePictureInPicture")
}, true)