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 perform a Solid Element Operation using the Morph and the Mesh, namely with the not-so-often used "Intersection" method, where the Mesh acts as Operator(!) and the Morph as Target.
After this operation, visit the Floor Plan. If the Morph had a Cover Fill enabled, it will show the cut geometry exactly where you need it – above the cut line.