If you click the left and right mouse button simultaneously on a "flat button" (also known as toolbar), the player interface will get locked, i.e. you can click anywhere anymore, until you switch to another application and switch back to OpenMPT again. Note that this does not happen with the main toolbar in the upper part of the windows since it has a context menu. however, it happens with the toolbars on the song tabs... And it also does not seem to happen with all OSes (or probably MFC versions?), since I cannot reproduce this under Win7 anymore.