Hello! This bug hit me too so I downloaded the Debian source package and managed to find which patch it was that caused it. When I recompiled Openbox without 'rpi/de67618ea9e3e22e7623d56530934d3a3fc272e0.patch' the problem disappeared.
> From de67618ea9e3e22e7623d56530934d3a3fc272e0 Mon Sep 17 00:00:00 2001 > From: spl <si...@raspberrypi.org> > Date: Fri, 11 Aug 2017 14:12:54 +0100 > Subject: [PATCH] Remove ability to maximise a window in a single direction. As indicated by the name of the patch it seems like the ability to maximise vertically or horizontally was intentionally removed, which I guess migh make sense in some configurations (eg. with limited screen size.) For the general case I beleive this patch should not be applied though. Cheers, -- Jonas Linde <jo...@init.se> - http://jonaseel.se/ - +46-707-492496