Found in X.org commit:

 commit d5bf6f95f31037bd49b11348b500c3c13b7e0c99
 Author: Keith Packard <keithp@keithp.com>
 Date:   Thu Oct 4 14:42:37 2012 -0700

    Fix FlushClient to write extraBuf when provided (regression fix)

    In commit:

        commit 092c57ab173c8b71056f6feb3b9d04d063a46579
        Author: Adam Jackson <ajax@redhat.com>
        Date:   Fri Jun 17 14:03:01 2011 -0400

            os: Hide the Connection{In,Out}put implementation details
Reviewed-by: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Adam Jackson <ajax@redhat.com>

    the check for an empty output buffer was moved from one calling
    location into the FlushClient implementation itself. However, this
    neglected the possibility that additional data, in the form of
    'extraBuf' would be passed to FlushClient from other code paths. If the
    output buffer happened to be empty at that time, the extra data would
    never be written to the client.

    This is fixed by checking the total data to be written, which includes
    both pending and extra data, instead of just the pending data.
Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Julien Cristau <jcristau@debian.org>
Backported-to-NX-by: Mike Gabriel <mike.gabriel@das-netzwerkteam.de>