you mean a matrixctrl? that would be the only mouse-controllable matrix-like interface so i'm assuming you mean matrixctrl but i don't find this problem, myself. perhaps your using it to control matrix~ in a strange way. post your patch and people can try it.
I allowed my self to attach the bug report , if one of you guys could help me out !!! that would really make my month , cos so far Max has been crashing to much (to the point that I would not even consider using it in a live gig situation , and that says a lot !)