Connection screen Lightdm

I have the following problem : following installation of a new screen (asus pa278qv) I have now during start up in the axis a the screen a message " display port not activated" during some seconds ; unfortunately screen connection ( configured with a password) windows is also displayed in the axis of the screen but behind , and then I have to wait few seconds prior to enter username and password ! I installed lightdm-gtk-greeter-settings to try to configure position of the connection screen but if all menu are present ( I can decide position of the window relative to top,bottom,right or left … ) but it’s not operative : window is still in the axis .
Then how is managed connection windows position in EOS , as it’s not through lightdm-settings?
Note : EOS artemis - xfce4 - Dell3910