I had same problem when running sweethome3d on Ubuntu and fixed it: Which gave me file /usr/share/sweethome3d/sweethome3d.sh which is used to start sweethome3d. I read that file and removed following line: It fixed my problem, I was able to enable top view after this change.

So maybe when everything is OK but swh3d does not check "off screen support" it does not allow to enable that feature.