Specify how the convex select mode is meant to work, to fix edge cases

### 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_