This is Grendel -- a Java mail/news client.

The grendel.view package is what gathers together the messages in a
Folder, calculates a sorting/threading order on them, and allows for a
UI to present them.

Much of the UI work is in the "thread list" -- a scrollable list of
messages, possibly organized in a threaded tree, which the user can
scroll through and select and drag and manipulate.  Most of the smarts
of the "thread list" are really implemented here in the grendel.view
package.

See http://www.mozilla.org/projects/grendel/ for more info.