gwibber uses different scrollbars in different content panes

In the content pane on the left, the overlay scrollbars are used, and work correctly.
In the content pane on the right, the classic scrollbars are used.
PS. The message input pane at the bottom uses the overlay scrollbars also (not visible in screenshot).

liboverlay-scrollbar-0.1-0:
  Installed: 0.1.4-0ubuntu1
  Candidate: 0.1.4-0ubuntu1
  Version table:
 *** 0.1.4-0ubuntu1 0
        500 http://ppa.launchpad.net/ayatana-scrollbar-team/release/ubuntu/ natty/main amd64 Packages
        100 /var/lib/dpkg/status

gwibber:
  Installed: 2.91.92-0ubuntu1
  Candidate: 2.91.92-0ubuntu1
  Version table:
 *** 2.91.92-0ubuntu1 0
        500 http://de.archive.ubuntu.com/ubuntu/ natty/main amd64 Packages
        100 /var/lib/dpkg/status