Specify how the convex select mode is meant to work, to fix edge cases #2817
Description
The Problem
Currently the convex selection mode is complicated, and not documented in exactly how it's meant to work. There are inconsistencies/oddities in the behaviour, but that's just "how it's always worked."
A Solution
If we come up with a strict specification, we'll be able to ensure that behaviour keeps to that spec.
Alternatives
No response
Anything Else?
No response