Originally Posted By: argv0
The expected behaviour is now to highlight the active mIRC window when mIRC itself is not the active application regardless if it is the active *mIRC* window.

I'm not sure if I understood that part right, but if I do, we are talking about different extents of this issue: I am aware that mIRC's active window being highlighted when mIRC is inactive is intended; this was discussed in another thread that RoCk linked in this one on page 1.
The issue I've been on about here the whole time is that that highlight is not removed when mIRC becomes active again in certain situations; namely if some other window stole focus from it on its own.
As said on page 1: If you manually open some smaller window over mIRC (like Winamp), you can see mIRC's active window being highlighted on activity in the background. When you close that other window, mIRC becomes the active app again and that highlight is removed. I think that's how it was meant to be. But: If an IM or something else you didn't initiate yourself pops up over mIRC and you close that, mIRC still becomes active again but the highlight stays.