YTLitePlus/Extensions/OpenYoutubeSafariExtension.appex/content.js

browser.runtime.sendMessage({ greeting: "hello" }).then((response) => {
    console.log("Received response: ", response);
});

browser.runtime.onMessage.addListener((request, sender, sendResponse) => {
    console.log("Received request: ", request);
});

function afterNavigate() {
    if ('/watch' === location.pathname) {
        window.location.href = `youtube://${window.location.pathname.slice(1)}${
            window.location.search
        }${window.location.hash}`;
    }
}
(document.body || document.documentElement).addEventListener('transitionend',
  function(/*TransitionEvent*/ event) {
    if (event.propertyName === 'width' && event.target.id === 'progress') {
        afterNavigate();
    }
}, true);
// After page load
afterNavigate();