how to check when active window changes

so I'm trying to write an app that needs to know when the window focus changes.. and track what window has focus and for how long..

so right now I had a timer checking every second to see what process was active and run that way.. but I ran into
an issue with firefox and I assume all browsers..

when I moused over like a quicklink at the top it gave a popup and then my code thought that was a new app.. but
really its just part of the main app.. so how in a browser would I know that this popup is really just part of the
tab at the top for this browser. I dont see a child property.. or maybe there is..

then also.. is this even the most efficient way to know the window has changed.. I assume some windows
event must fire when the window changes.. I guess I could use that to know ??

any pointers or ideas are appreciated..