Regarding non-Alt-Tabbable, excluded from Overview popups: according to gnome-shell developers on IRC, it's because _NET_WM_STATE includes _NET_WM_STATE_SKIP_TASKBAR.
Regarding non-Alt-Tabbable, excluded from Overview popups: according to gnome-shell developers on IRC, it's because _NET_WM_STATE includes _NET_WM_ STATE_SKIP_ TASKBAR.