If you open sddm-kcm if won't load until you resize the window. Also if you make changes the changes aren't shown immediately until you move out of the kcm. Please see the attached screencast. Reproducible: Always Steps to Reproduce: 1.SystemSettings>Startup and Shutdown>Sddm 2. 3.
The video was too big. Here is the YouTube version. https://youtu.be/t_YEEejbREo
Have the same problem. Cannot use Login Screen (SDDM) configuration from system ssettings in Fedora 23.
This has been fixed in Fedora 23 with the sddm-kcm-5.5.4-1.fc23.x86_64 package.
Marking as fixed as noted on the last comment.