I ran into a surprising situation when closing the default tab with a message tab being the only one left. In this case, it is not obvious for the user how to obtain access to the folder and thread panes again. Steps to reproduce: 1) Open mail/news window in default view, single tab, no tab bar. 2) Select "Open Message in New Tab" from context menu. 3) Close the default tab, the message tab is the only one which remains. 4) Try to get the folder and thread panes back: *) Ctrl+T opens a clone of the current tab; *) View > Show > Location Bar gives a folder menu but no pane. 5) Close and reopen mail/news or restart, still no folder pane present. With View > Layout > Folder Pane / Thread Pane, the situation can be resolved, but is quite confusing. The suggestion is to prevent the default tab from being closed (as is the case in its Thunderbird counterpart) so that a stable point of entry is retained.

You can use View->Layout->Message Pane/Folder Pane/Thread Pane or F8/F9/shift F8 resepctively to unhide missing panes. There's no solid reason in making the first tab special -> WONTFIX.