It seems I can't reproduce this bug. Exactly which widget, the one which displays the mails threaded and all?, or the one which displays the folders? it seems I can navigate with the arrows in both of them (although the folders one with inconsistent behavior).

> Exactly which widget, the one which displays the mails threaded and all?

Yes, the message list widget, which displays the mails threaded. It does not make a difference which widget is focused, the keyboard shortcuts should always work.

> it seems I can navigate with the arrows in both of them

That should actually not be possible :)
Left arrow should go to the previous email
Right arrow should go to the next email
Up arrow should scroll the message viewer upwards
Down arrow should scroll the message viewer downwards
+ should go to the next unread message
- should go to the previous unread message

Actually left/right seems to be working :)
I couldn't check going to the next/previous unread message, since the marking of messages doesn't work reliable.

So this bug report might actually be bogus, I don't know...

I think this is a bogus report.