Comment # 4 on bug 1082713 from Fabian Vogt

(In reply to Terje J. Hanssen from comment #3)
> Ok, ssdm is the name of the login menu that remains to the Plasma desktop.
> 
> I just tell how this has changed vs previous experience/distributions.
> 
> Before Leap 15 when one selected Gnome as default system desktop during the
> installation, it resulted in GDM login menu with Gnome as default desktop
> session (the latter is user change-able at login of course), also when the
> KDE/Plasma software pattern was added during the initial installation (and
> vice versa).
> 
> Now with Leap 15 the same selections result in and boot up the 'ssdm' login
> menu and Plasma as the default desktop session (the latter user changeable
> at login).

Yes. The desktop selection mechanism in Leap 15 got changed.

> As I mentioned, to get and preserve the GDM login menu, I had to do the
> installation i two-steps, Gnome desktop only in the initial installation,
> and in a second installation step afterwars add the Plasma pattern.

Just run "sudo update-alternatives --config default-displaymanager" and "sudo
update-alternatives --config default-xsession.desktop" on the first start.