Why does not keypresses to the pause/break key reach `xev`?

Question

I'm running xfce and as far as I can see the pause key is not mapped as a hotkey. When pressing the pause key it only produces:

FocusOut event, serial 37, synthetic NO, window 0x3000001,
    mode NotifyGrab, detail NotifyAncestor

FocusOut event, serial 37, synthetic NO, window 0x3000001,
    mode NotifyUngrab, detail NotifyPointer

FocusIn event, serial 37, synthetic NO, window 0x3000001,
    mode NotifyUngrab, detail NotifyAncestor

KeymapNotify event, serial 37, synthetic NO, window 0x0,
    keys:  2   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   

and it doesn't matter if I remap the pause key to something completely different, for example:

    key <PAUS> {
//  type= "PC_CONTROL_LEVEL2",
    symbols[Group1]= [           p,           Break ]
    };

If I hit Shift+Pause I get response:

KeyPress event, serial 37, synthetic NO, window 0x3000001,
    root 0x1ef, subw 0x0, time 679325549, (37,115), root:(1228,774),
    state 0x0, keycode 50 (keysym 0xffe1, Shift_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyPress event, serial 37, synthetic NO, window 0x3000001,
    root 0x1ef, subw 0x0, time 679325877, (37,115), root:(1228,774),
    state 0x1, keycode 127 (keysym 0xff6b, Break), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 37, synthetic NO, window 0x3000001,
    root 0x1ef, subw 0x0, time 679325901, (37,115), root:(1228,774),
    state 0x1, keycode 127 (keysym 0xff6b, Break), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 37, synthetic NO, window 0x3000001,
    root 0x1ef, subw 0x0, time 679326061, (37,115), root:(1228,774),
    state 0x1, keycode 50 (keysym 0xffe1, Shift_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

Why does not the unshifted pause key reach xev?

Answer 1

The FocusIn and FocusOut events are a sign that some other application (possibly the window manager, possibly some other part of your desktop, possibly a completely different application) is stealing these events, because it has registered interest with the X server for these particular keys.

So while they may not be "mapped as a hotkey" in xfce, some application still tries to react to them.

The obvious way to debug this is to stop/kill applications until you see the events again in xev. The output of xlsclients should help finding out candidates.