On Thu, 29 Dec 2016, Steve Litt wrote:

> The host uses Openbox. Next time I shut everything down and restart
> X, I'll try it with Xfce and fvwm ON THE HOST and see if that
> changes things.

what can be also useful for the purpose of troubleshooting, use:

xev -event keyboard

and then press and release ctrl-alt inside the host wm, what does that
prints out? I run Devuan Jessie on i3 on kernel 4.7.0-0.bpo.1-amd64:

KeyPress event, serial 28, synthetic NO, window 0x1c00001,
    root 0xa3, subw 0x0, time 189416334, (205,375), root:(892,399),
    state 0x0, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyPress event, serial 28, synthetic NO, window 0x1c00001,
    root 0xa3, subw 0x0, time 189416334, (205,375), root:(892,399),
    state 0x8, keycode 66 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x1c00001,
    root 0xa3, subw 0x0, time 189416898, (205,375), root:(892,399),
    state 0xc, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x1c00001,
    root 0xa3, subw 0x0, time 189416915, (205,375), root:(892,399),
    state 0x4, keycode 66 (keysym 0xffe3, Control_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

ciao