I have a long text and I'd like to offer the user a reading help: The current line should be highlighted. To make it easier, I'll just use the Y coordinate of the mouse (this way, the mouse pointer isn't going to get in the way). I have a big DIV with the id content which fills the whole width and a small DIV with the class content for the text (see here for an example).
I'm using jQuery 1.4. How can I highlight the line of text that is closest to the current mouse position?
Not sure if jQuery will help you out much here, but you could take a look at the element.getClientRects method, documented on MSDN and MDC. More specifically, this example at MSDN is sort of similar to what you want to achieve, highlighting lines using a cleverly z-indexed div element that goes behind the text at the co-ordinates returned by getClientRects().
You should be able to achieve the same thing by looping through the TextRectangle objects returned in the document's onmousemove and checking to see if the y value of the mouse cursor is > the top and < the bottom of each rectangle and moving the cleverly z-indexed div to the same position/height.
All the current major browsers support getClientRects().
UPDATED - working in Chrome, IE6/7/8, Firefox, Opera, Safari. The initial problems I had in the other browsers were related to the DIV needing to be display: inline.
UPDATED AGAIN - I had to refer to this answer for some newer questions, so I took the time to update it to recalc the lines on window resize. It looks like others have been playing around too, it's now on revision 15.