I use this code How can I position an element next to user text selection? to get the position of the selected text, but it doesn\'t work for the selected text inside an input.
If you just need to get the position where the user doubleclicked, use the following snippet.
$('#thatInput').on('dblclick', function (e) { alert('Position X: ' + e.clientX + '\nPosition Y: ' + e.clientY); });