You can magic-wand a zone stamp to place a polygon using the geometry of the zone itself. It works even if the zone polygon is hidden, and saves you the trouble of finding an edge, since zone edges tend to coincide with lots of other stuff. Magic-wanding the middle of a room is often impractical due to interference of other lines.
Handy for finish-floor slabs, lower-the-ceiling-in-one-room slabs, floor material fills, etc. Just make sure your zones are updated first.
Cross-posted here.