I would like to select part of a mesh using a plane as a "boundary indicator".
In the screenshot below that would be the mesh part above the plane:
I can get the desired selection by using Bisect tool (Bisect, hide, select a vertex, select linked, unhide):
however, this is visually not as convenient as using a plane (this is only one step in a longer workflow which has to be simplified). Beside that, Bisect cuts trough the whole object, what is inconvenient (notice the bisect-line cutting another part of the mesh in lower right).
I know I can use Boolean modifier to get only the part above the plane, but I do not want to cut it off - I just want to select it for editing.
The "transition line" does not have to be straight (i.e. adding vertices and edges), it can use existing edges (resulting in a jagged "transition line").
What I meant is to use the plane in a similar way it would be used in Boolean modifer, just not to "cut off" but to select that part of the mesh.
The goal is to automate the process, e.g. operator positions the plane and clicks on a button (to run a script that does the rest; I can write the script once I know the steps).