Comment 7 for bug 981628

I can confirm that this happens here, too. The xev output shows only

KeymapNotify event, serial 33, synthetic NO, window 0x0,
    keys: 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
           0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

when left Alt is pressed and Num Lock is on, but

KeyPress event, serial 33, synthetic NO, window 0x4400001,
    root 0xc4, subw 0x0, time 85846911, (1170,1080), root:(1171,1160),
    state 0x0, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes:
    XmbLookupString gives 0 bytes:
    XFilterEvent returns: False

KeyRelease event, serial 33, synthetic NO, window 0x4400001,
    root 0xc4, subw 0x0, time 85846974, (1170,1080), root:(1171,1160),
    state 0x8, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes:
    XFilterEvent returns: False

when Alt is pressed while Num Lock is off.