SUMMARY
On SDDM login screen using Breeze/Breath theme, the login module would disappear after a while without input. Pressing Enter does not reveal the password input field but actually submit the password. This is a small thing but nonetheless a real nuisance. Combined with SDDM not being able to show masked password letters on some occasions (on multiple screen setup) it is a major source of confusion.

This bug was first reported to SDDM: https://github.com/sddm/sddm/issues/1089 And with David Edmundson pointing it out, we relay it to Plasmashell/Lookandfeel

My opinions:

People usually make their input only when they can see an input field. They probably do not realize they are still inputting password when the password field is absent. Unwanted letters would probably be recorded as part of the password and cause the login to fail, seemingly "without any reason".

People tend to use mouse click or to press a key on their keyboard (usually a modifier/space/enter) to reveal password fields before they consider themselves in a valid password input conversation. This is also how roughly Windows 7+ and many other OS design their login screen.

Although we can move mouse or input letters to reveal the login module, note that clicking a mouse button does not always comes with mouse movement, so it is very confusing as the user would perceive it as "the password field appears only sometimes as I click my mouse".

My suggestions:

1) Allow modifier/enter/space/mouse click/drag to reveal password input module.
2) When the login module is hidden, do not record any password input.
3) When the login module is hidden, do not submit the password by pressing ENTER.
4) Always show correct masked letters when the password input field is on screen.

SOFTWARE/OS VERSIONS
Windows: 
MacOS: 
Linux/KDE Plasma: Manjaro 18.0.2 KDE
(available in About System)
KDE Plasma Version: 5.14.4
KDE Frameworks Version: 5.53.0
Qt Version: 5.12.0