If you're using the Last.fm recommmendations, it might be this bug, which is fixed in git master :On Tue, 2009-05-05 at 20:04 +0200, David Nielsen wrote:
>
>
> 2009/5/4 Apostate <apostate diebruecke us>
>
> ....with each restart. I am on Ubuntu 9.04. If I quit Banshee
> completely, and
> then restart it, the Library pane is unusually large. If I
> repeat this
> without dragging it back in, it will eventually force the
> right-most pane
> right off the screen!
>
> Anyone else seeing this?
>
> I can confirm this, it is annoying but not fatal. It started with
> 1.4.2 I believe.
http://bugzilla.gnome.org/show_bug.cgi?id=578896
If not, please file a new bug.