normal 3 pane layout. without a default, the splitter between the thread pane and the message pane jumps when we switch folders, which looks bad and causes a repaint, which is bad for folder loading performance. r=cavin, sr=bienvenu git-svn-id: svn://10.0.0.236/trunk@105531 18797224-902f-48f8-a5cc-f745e15eee43