The openbox package in Debian does not support startup notifications and
such behavior creates big problems, especially on slow machines, tablets
and devices like Raspberry Pi, because users cannot see if application
they tried to start really started, and in some cases duplicated try may
freeze whole system (if memory exhausted for example, or other resourse
locked) so cold reboot is needed. Thus may be severity of this bug should
be "serious" but "important" is the least.
Fix is very much trivial though - just adding libxcursor-dev to build
dependencies will fix the issue. Please, fix it both in jessie and in
sid. Thank you.