Thanks, that was fast! Here's an idea on how to implement it without the slowdown: https://jsfiddle.net/qpswztj8/ https://jsfiddle.net/qpswztj8/ Given that it

Thanks, that was fast! Here's an idea on how to implement it without the slowdown: https://jsfiddle.net/qpswztj8/ https://jsfiddle.net/qpswztj8/ Given that it's a preformatted text with a known number of columns, the number below the mouse pointer can be computed using the mouse position, character width and line height.