If I'm reading this correctly, you cannot reword a commit without first checking it out, and you cannot preview merge conflicts without first starting a merge/rebase operation? Those are the criteria as described at https://blog.waleedkhan.name/git-ui-features/#rubric

Reword: you don't need to check out the commit. The 'log' tab shows a tree of the commits to the repo. This log can be filtered in multiple ways: for example by branch or via a search string. You may right click a commit to reword.

Thanks. I confirmed that you can reword a commit which is reachable from `HEAD`, but not those on other branches in this way, similar to Sublime Merge elsewhere in the comments.

To confirm we're not taking past each other: You can change 'Branch' to 'All' to see everything, and then re-filter.

I right-clicked commits not reachable from `HEAD` and the Edit Commit Message option was disabled, so I'm assuming it can only reword commits reachable from `HEAD`.

I profusely apologise, I was incorrect.