Add new scriptlet — 'disable-pageview-api'

YouTube uses pageview api to show popup "Video paused. Continue watching?" when user is not interacting with the page.

/// disable-pageview-api.js
(function(){
// based on https://addons.mozilla.org/en-US/firefox/addon/disable-page-visibility/
// visibilitychange events are captured and stopped
document.addEventListener("visibilitychange", function(e) {
e.stopImmediatePropagation();
}, true);
// document.visibilityState always returns false
Object.defineProperty(Document.prototype, "hidden", {
get: function hidden() {
return false;
},
enumerable: true,
configurable: true
});
// document.visibilityState always returns "visible"
Object.defineProperty(Document.prototype, "visibilityState", {
get: function visibilityState() {
return "visible";
},
enumerable: true,
configurable: true
});
})()

Discussion on reddit regarding it:
https://www.reddit.com/r/firefox/comments/f0ewaq/any_way_to_disable_pageview_api_via_unlock_origin/