OpenBox lacks support for _NET_WM_FULLSCREEN_MONITORS. This was discussed on bug 5182, but that bug was about problems on Thinstation and is now closed.
The lack of support for _NET_WM_FULLSCREEN_MONITORS means that you cannot run any applications with tl-single-app which wants to go fullscreen over multiple monitors. This could be vncviewer/tlclient, x11clone, FreeRDP or some other application.
Upstream bug: https://bugzilla.icculus.org/show_bug.cgi?id=6246
(In reply to comment #0)
> Upstream bug: https://bugzilla.icculus.org/show_bug.cgi?id=6246
More than 4 years has passed, but unfortunately no progress.
According to my experience with common WMs, "xfwm4" is the only WM that works correctly. Perhaps we should consider shipping that one instead?
This was tested again as part of bug 7006, comment 46, and running applications in fullscreen is still not supported.