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			Сообщение от ekad
			
		
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		к каждому новому созданному div добавить id
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<!DOCTYPE HTML>
<html>
<head>
    <title>Untitled</title>
    <style type="text/css">
        [contenteditable] > div.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 = () => {
            edit.querySelectorAll('.active').forEach(div => div_active === div || div.classList.remove('active'));
            document.querySelectorAll('#pt_text > div').forEach((div, i) => div.id = `n${++i}`);
        };
        deactivate();
        document.addEventListener('selectionchange', () => {
            if (document.activeElement === edit) {
                let el = window.getSelection().focusNode;
                if (el.nodeType === 3) el = el.parentNode;
                div_active = el;
                deactivate();
                div_active.classList.add('active');
            }
        });
        edit.addEventListener('focusout', ()=>{div_active=null,deactivate()})
        edit.addEventListener('keyup', deactivate)
    </script>
</body>
</html>