This looks like normal behaviour to me. If a window is full size and the treebar (or any other bar) is enabled, the MDI window area becomes smaller, so existing windows will extend beyond the MDI window area. mIRC has always behaved this way as far as I know. I could resolve it by making mIRC reposition and resize all of your windows to fit into the MDI area every time you change a display setting, however I don't think that would be desirable behaviour.
