Function shell_lost 

pub fn shell_lost() -> bool

The shell hosting this window went away. A shell says where the workspace stands in its first message to a window (every shell does: the app and the shell speak one shell-ui version, or do not connect), so a window still waiting for that word never gets it: what the workspace holds cannot arrive, and the window says so (WorkspaceUnreachable::ShellGone). Whether that changed what the window shows.