Hi Hans,

Thanks for the prompt reply.

The room was created manually (not double-click), but the problem persists. Indeed, as you say, the room, which is actually a hall in the second floor, has "walls" that are not of the same height. Some of the walls are real walls, and some are a low metal railing. I needed to put floor beneath the railing, so I included the railing area in the room (by disabling magnetism first).
However, the actual inclusion of the railing (i.e., putting the right of the two topmost corners beyond the railing) causes the triangle to appear - how can this be avoided?

You mentioned another method involving a box to get rid of the triangle - can you please elaborate?

Regards,
Erez