Please read carefully, I think _all_ the ingredients I list here are
needed to reproduce this bug.

I start with my folder list collapsed, ie. only top level folders are
shown.  (In my server's IMAP implementation top level folders are
siblings of INBOX, not its children.)  I open one of the top level
folders (INBOX should do) and interact with it, reading some messages
and switching among messages with the Up and Down keys.  Then I decide
to open another folder, but one that is nested 1 level deep and
therefore not displayed in the folder list.  To that end I hit the "Go
To Other Folder" key combo (Ctrl-G for me).  I select and open the
desired folder.  Claws has helpfully expanded the folder list so that
the newly opened folder is now visible and highlighted in the list.
Again I interact with the folder and its messages, using Up and Down.

Finally I hit the Left key, giving focus to the folder list (or so I
hope - the visual focus representation in Claws is so weak that I can't
say for sure, but the folder list seems to react to keystrokes in
general).  And I have arrived in bug land: the Up and Down keys do not
work.  The Up key does nothing at all, and the Down key highlights the
topmost line in the list, ie. the account line.

I think I know what is happening here:  the folder list keeps state
including the highlighted line but this state is not updated correctly
when a parent folder is expanded as a side effect of selecting one of
its children in the Go To dialog.