I expect that this is a (low-priority) bug, but apport-bug strongly encouraged me to come here first, so here I am...
What I did: I started Thunderbird and then minimized the window. I
then middle-clicked on the Thunderbird icon in the Unity (3D)
Launcher. I do not have any of the appmenu packages installed (not
indicator-appmenu, nor any of the *-globalmenu or appmenu-* packages).
What I expected: I would get the Thunderbird main window back at its
original location, or possibly I'd get a Compose Mail window somewhere
on the desktop. (This was something of an experiment, so I wasn't
really sure what to expect.)
What happens: The Thunderbird main window appears in the upper left
corner of the display, displacing the Launcher. This was not its
previous location. The window has no titlebar or menubar. The top
panel says "Thunderbird Mail", but moving the mouse over that text does
nothing (doesn't show the close/minimize/maximize controls). I can
still bring up the Launcher and start applications. If I start
Firefox and give it input focus, clicking on the Thunderbird window
leaves the focus with Firefox. I can use the Switcher to give
Thunderbird the input focus. (Both the Unity Switcher and the Static
Application Switcher work. If I use the Static Application Switcher,
I see Thunderbird's menubar in the top panel until I release Alt-tab.)
I can kill Thunderbird from the Launcher. I can also use the Unity
Switcher to minimize everything. If I then left-click on the
Thunderbird icon in the Launcher, the Thunderbird main window
reappears in the upper left. But this time it does not displace the
Launcher, and it has the proper titlebar and menubar.
This does not happen with Unity 2D. And I haven't seen it with any other app.
I realize that because I've disabled the appmenu stuff, I'm not getting the full Unity experience, and there might be some rough edges. But this is a bug, yes?
