M-SPACE on linux opens menu vs. just-one-space

[Peter Lee]

M-SPACE on linux opens system menu at the top left of frame instead of
running the command just-one-space.  I don't have this problem on
winXP and I have a feeling it's my window manager getting in the way,
but I have no idea where to look.  Anyone ideas?  I was thinking I
need to add something to my .Xdefaults, but not I'm just grasping.

I know I can get around this by doing ESC-SPACE, but I use that
binding a lot and would prefer to use the M-SPACE.

Thanks.