gtk_application_impl_dbus_startup() calls g_unsetenv(). Here be dragons. As per the documentation of g_unsetenv(), and experience (e.g. bug #754951), this will explode randomly. g_unsetenv() may only be used before the first secondary thread is created, but GApplication starts working with GDBusConnection at GApplication register time, before GApplication startup, and certainly well before GtkApplication's startup.

This will be difficult to fix. One option is to document that the application must not create any threads before calling g_application_run(), and hope that applications get that right (they will not). Then store the desktop ID as global state way earlier than startup. I think GtkApplication class init would be a "good" place, since it's class data and not instance data. Alternatively, this could be done using a library constructor, which would make it very heard for applications to create a thread too soon, except then we'd have to leave Windows broken.

Another option would be to give up on unsetting the environment variable, and modify g_spawn() to always pass a custom envp to the child process with it removed. That would work so long as a GApplication always uses g_spawn() when spawning another GApplication. Eh.