Comment 18 for bug 1626651

Okay, an interesting hint is that this problem seems to be fixed if u-s-d is started under the Upstart user session instead of the systemd user session.

If anyone wants to try this, just comment out these three lines in /etc/X11/Xsession.d/00upstart:

    if [ "${1#*.target}" != "$1" ]; then
        export XDG_CONFIG_DIRS="/usr/share/upstart/systemd-session:$XDG_CONFIG_DIRS"
    fi

I'm attaching the result of me starting u-s-d under Upstart, running `sudo forkstat -D 4`, and then pressing the brightness up key.

(I'll attach the same from under systemd next comment.)

From talking to Martin Pitt on IRC, sounds like switching u-s-d back to being started by Upstart might not be viable, but hopefully this at least helps narrow the search.