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

[me4502]

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