Since a11y navigation was added for all RHS panels in 5.15.2 (in addition to the already supported thread comments) navigation order from post to post has not been correct but instead has been jumping up/down seemingly randomly but only when using Desktop App v4.2.3 or below.
This can be seen on community.mattermost.com by doing a search for the word 'test' while using Desktop app v4.2.3. This can also be repro'd in a thread if there are a bunch of comments from various people with several [at]mentions – not sure if the [at]mentions play a role or not however.
Using Desktop App v4.2.x
1. Perform a search for a topic that is sure to return a bunch of posts
2. Use keyboard to navigate to RHS (F6) – top result should be highlighted first
3. Use up/down arrow keys to navigate from one result to the next – navigation should be sequential and not skip around randomly, navigation should *not* loop
3. Open up a *long* thread with multiple participants and [at]mentions
4. Use keyboard to navigate to RHS (F6) – bottom comment should be highlighted first
5. Use up/down arrow keys to navigate from one result to the next – navigation should be sequential and not skip around randomly, navigation *should* loop