ekad,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<style type="text/css">
[contenteditable] .active {
border: 4px red solid;
}
</style>
</head>
<body>
<div id="pt_text" contenteditable="true">
<div>111</div>
<div>111</div>
<div>(тут курсор)</div>
</div>
<script>
let edit = document.querySelector('#pt_text'),
div_active;
const deactivate = () => div_active && div_active.classList.remove('active');
const activate = div => {
deactivate();
div_active = div;
div_active.classList.add('active');
}
edit.addEventListener('click', ({
target
}) => {
if (taret = target.closest('#pt_text > div')) activate(target);
});
edit.addEventListener('focusout', deactivate)
</script>
</body>
</html>