window.addEventListener('load', (event) => {
    window.webkit.messageHandlers.contentLoaded.postMessage("content has loaded");
});

window.addEventListener('DOMContentLoaded', (event) => {
    window.webkit.messageHandlers.contentLoaded.postMessage("content has loaded");
});

mounted() {
    window.webkit.messageHandlers.contentLoaded.postMessage("content has loaded");
}

mounted() {
    window.webkit.messageHandlers.contentLoaded.postMessage("content has loaded");
}

// This works!
button.addEventListener('click', () => {
   window.webkit.messageHandlers.contentLoaded.postMessage("content has loaded");
})