Problem: 
After toggling on a group's "Show only new articles" filter from the
toolbar button, switching to another group, then returning, clicking the
toolbar button again will change its visual state to unpressed, but will
not untoggle "Show only new articles".  "Filter->Match New Articles" from
the menu will fix this.

Procedure:
1) Choose any group and download new headers.
2) Press the "Show Only New Articles" toolbar button to toggle the "New
Article" filter on.
3) Choose any other group and download headers.
4) Switch back to the group selected in (1).
5) Press the (now active) "Show Only New Articles" toolbar button to
untoggle.  The visual state of the button changes to off, but the display
of articles does not change.