Mouse clicks "delayed" until movement

[Stefan Monnier]

Since last reboot, my trusty laptop has the following annoying behavior:
mouse clicks don't take effect until the next event (typically moving
the mouse).

I checked with `xev` and indeed after a mouse click I see the following
output:

    [...]

    MotionNotify event, serial 35, synthetic NO, window 0x3800001,
        root 0xf6, subw 0x0, time 21834559, (83,68), root:(369,570),
        state 0x0, is_hint 0, same_screen YES
    
    ButtonPress event, serial 35, synthetic NO, window 0x3800001,
        root 0xf6, subw 0x0, time 21836087, (83,68), root:(369,570),
        state 0x0, button 1, same_screen YES

and it sits there without showing any "ButtonRelease" even though
I definitely released the mouse button.  When I then press a key
I finally see the ButtonRelease followed by the key press/release:

    ButtonRelease event, serial 35, synthetic NO, window 0x3800001,
        root 0xf6, subw 0x0, time 21837777, (83,68), root:(369,570),
        state 0x100, button 1, same_screen YES
    
    KeyPress event, serial 35, synthetic NO, window 0x3800001,
        root 0xf6, subw 0x0, time 21837777, (83,68), root:(369,570),
        state 0x0, keycode 111 (keysym 0xff52, Up), same_screen YES,
        XLookupString gives 0 bytes: 
        XmbLookupString gives 0 bytes: 
        XFilterEvent returns: False
    
    KeyRelease event, serial 38, synthetic NO, window 0x3800001,
        root 0xf6, subw 0x0, time 21837880, (83,68), root:(369,570),
        state 0x0, keycode 111 (keysym 0xff52, Up), same_screen YES,
        XLookupString gives 0 bytes: 
        XFilterEvent returns: False

Any idea where the problem might be coming from, how to track it down,
or (even better) fix it?

This is a Thinkpad T61 running Debian testing, with XFCE running
on Xorg.


=== Stefan