Issue Description
In the plan window, how do you show the cut surface of a mesh?
Solution
Archicad has no direct solution for this. There is a very neat workaround, that is quite simple to follow.
Take a Morph and draw a big rectangle (bigger than your mesh) in the floor plan.
Then set the morph at the right height where you want to cut.
Now SEO the morph and the mesh, namely with the not often used "Intersection" method where the mesh acts as Operator (!) and the morph as Target.
Now, if your morph has a cover fill, it will only be visible exactly where you need it.