Window Maker does not call the RegisterSession() method on GDM's D-Bus
interface, causing that GDM doesn't know that the login was successful,
which leads to problems. If X-GDM-SessionRegisters is not specified or
false, GDM registers the session itself.
I noticed this when I logged out from Window Maker, and tried to log
in into another session with GDM, it doesn't work, because the login
screen hangs as the previous session was not entered into registered
state within GDM.
If X-GDM-SessionRegisters=true is specified, GDM expects that the
session will be registered via D-Bus:
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E.g. GNOME Shell calls RegisterSession() method after login:
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Older versions behave the same way as would be X-GDM-SessionRegisters
not set or false.
Currently X-GDM-SessionRegisters=true is used only by GNOME Shell. All
other sessions omit this property, because they don't call the
RegisterSession() method on GDM's D-Bus interface after login.
Files in /usr/share/xsessions are used by some display managers (e.g.,
LightDM and GDM) to detect available sessions. Such a file has been
shipped in the Debian Window Maker package for years.
