A modal dialog in the shell blocks anything but that dialog from
receiving user input. Applications within the session and other
parts of UI are rendered non-reactive.
When GDM gets changed to use the shell for its greeter, the user
list will be presented as a shell dialog. That dialog shouldn't
block access to the panel menus, etc.
This commit adds a shellReactive property that makes the ModalDialog
class continue to block access to applications, but allow the user
to interact with the shell itself.
https://bugzilla.gnome.org/show_bug.cgi?id=657082
db39ba3b9f