I've found a small but tedious problem with QMidi: if I choose to hide the menubar (from System Preferences), the transport bar appears and disappears almost randomly; it seems that moving the mouse the window may appear but this doesn't always happen.
So sometimes I cannot immediately start/stop a song because I cannot find the button to click.
I cannot say for sure if this is a operating system or QMidi problem: I know that the transport window is something like a secondary window, but these kind of window disappears only if its application goes to background, while this problem happens even if QMidi is the frontmost application.
If I make the menubar visible, the problem is gone (and this is my solution for now).
Just to be clear, this is the situation: