<style>
body {
    width: 100%;
    height: 100%;
}

#editable {
    width: 100%;
    height: 400px;
    font-size: 400px;
    white-space: nowrap;
    border: 1px blue dashed;
}
</style>
<div id="editable" contenteditable>Hello world</div>
<script>
document.addEventListener("selectionchange", () => {
    if (!getSelection().rangeCount || getSelection().isCollapsed)
        return;

    setTimeout(() => window.webkit.messageHandlers.testHandler.postMessage("selected"), 0);
});
</script>