create a hidden field lastfocus, and add a onfocus event to each control
that stores its id in the hidden field.

you approach is probably bad. when run locally on your desktop, the
repaint is quick, but remotely it will be seconds between when the
autopostback and re-render (your focus restore) happens.

you should switch to an ajax solution, or design to not use an autopostback.

-- bruce (sqlwork.com)