## Summary of the Pull Request
This is a spec for "pane navigation", as we've already got a bit of an implementation in #8183. We've also had a heated discussion in Teams, and I wanted to capture a bit of that in a more formal doc. I suppose that "informal Teams chat" didn't work out in the end 😆.
Also, this is @PankajBhojwani's feature so I'm gonna let him drive. I mostly wrote this to test out a new spec template.
After discussion, we landed on proposal D, with a minor change of `last` to `prev`. This is how it was in #8183 before I started meddling 😝
## PR Checklist
* [x] spec for #2871
* [x] I work here
## Detailed Description of the Pull Request / Additional comments
This is not my best spec ever - again, mostly just trying to spawn discussion, and prototype the new spec template.
bc70a97fd7