Setting for user identification together with password input in the same dialogue of a login manager

A login manager provides an interface for an authentication system. Now I would like to check once more which system settings influence the display if an user identifier can be selected together with a password input in the same dialogue.

I have got difficulties to find appropriate documentation for the configuration of a software component like “LightDM GTK+ greeter”. I would appreciate further advice.

Which content management system does store the original source files for the software component “LightDM GTK+ greeter”?