Way To Refresh A Function Upon Mouse Scroll

I have a home grown lightbox effect as my client cannot have community code such as JQuery, mootools etc running on their site (don't ask me....). All is working fine, the background darken mask with the box itself with the content. The only problem I'm having is what happens when the user scrolls.

As the mask is calculated upon clicking a link to display the lightbox, the method calculates the current screen area and uses that to create the height and width of the mask. But obviously if you scroll, you leave the mask behindat the top of the screen covering the calculated area. Is there a way to access the onscroll event to say rerun the specific function that calculates the mask area when the user scrolls so the mask stays constantly covering the available screen area? I suppose this is similar to those scripts where a div remains in view when the user scrolls - presumably something along those lines?

I found and tried a script to do with captured delta movement of the mousewheel but that disconnect the mouse wheel from the scrolling of the scroll bar and associated it only with the script in hand - I would like the browser to still scroll and my function to be fired on each scroll event.