On Linux with X11, Alt+Tab doesn't work while dragging an email out of Thunderbird, because GTK grabs the keyboard during the drag and the window manager never sees the key press.
I wrote a small open-source helper that works around this: it listens to raw XInput2 events and switches to the next window when Alt+Tab is pressed during a drag. In case it's useful to others:
https://github.com/Codatus/dragtab
It would of course be nicer if this worked out of the box – as far as I can tell it's GTK's drag-and-drop behaviour on X11 rather than something specific to Thunderbird.
