Having just migrated to a new MacBook Pro, which entailed moving from OS/X 10.11 to OS/X 10.13, I find that I can not type control-space at X windows. It works fine in other windows. Running xev, when I press control-space, I see the events for the control key press and release, but no events for the space bar press and release which happen while I'm holding the control key. Here is an example:

KeyPress event, serial 34, synthetic NO, window 0x1800001,
    root 0x1d1, subw 0x0, time 199551973, (92,61), root:(132,124),
    state 0x0, keycode 67 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 34, synthetic NO, window 0x1800001,
    root 0x1d1, subw 0x0, time 199554149, (92,61), root:(132,124),
    state 0x4, keycode 67 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False