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"A client indicates that it is willing to participate in the protocol by
listing _NET_WM_SYNC_REQUEST in the  WM_PROTOCOLS property of the client
window  and  storing  the  XID  of  an XSync  counter  in  the  property
_NET_WM_SYNC_REQUEST_COUNTER."

That's why I have only defined accessors for _NET_WM_SYNC_REQUEST_COUNTER.
But not for _NET_WM_SYNC_REQUEST  (only a ClientMessage function indeed)
as  it's   listed  in  WM_PROTOCOLS   on  a  client   window,  therefore
xcb_set_wm_protocols  from  xcb-util/icccm  should  be called  for  this
Atom. Does it make sense?

Thanks again for your review, it does help a lot.

Cheers,
Arnaud

[0] http://cgit.freedesktop.org/~arnau/xcb-util/commit/?h=3Dxcb-ewmh&id=3Df=
f5eae62cee4f13ae4df5f1bb58c62b605c03290
[1] http://cgit.freedesktop.org/~arnau/xcb-util/commit/?h=3Dxcb-ewmh&id=3Da=
e4e594200e41f809e07f1133bb04da5e2752ebb