Assume that you have a dual screen setup, say screen A and B. Assume that you configure the screens so that A is the primary screen (panel goes on A). Assume that you set things up so that A goes above B. If you have a window in A and you ask kwin to maximize (or to maximize it vertically), the maximization height is wrong. Kwin maximizes to the full screen height, rather than subtracting the panel height. If you set up things so that A goes below B then windows on A are maximized correctly. Hence I suppose that in multihead setups kwin forgets to check where the panels are before deciding how to maximize.
please have a look at the duplicate if that is your problem. If not please reopen. To me it sounds exactly like you have a panel between the two screens and this gets ignored. *** This bug has been marked as a duplicate of bug 167852 ***
Yes, I think you are right in marking duplicate status. Sorry for the noise.