Authentication through fprintd breaks the interface and is unusable

 * slick-greeter 1.3.2
 * Manjaro 20.1
 * Intel 2nd Generation Core Processor Family Integrated Graphics; `video-linux` I guess?
 * 64 bit

Since I am using fingerprint authentication through fprintd, I have added appropriate entries in /etc/pam.d/. But there are several issues with how slick-greeter handles such login methods:

• The output of pam module can easily overflow horizontally, resulting in unreadable messages.
• Several lines of output result in vertical overflow of new text as well as of the control elements – buttons, input fields, etc. Then it's impossible to determine if the fingerprint was accepted, if we the user is asked for another attempt or if the password login was activated.
• Successful authentication still requires user input, like clicking the login button or – especially if it overflowed – blindly doing so with keyboard.
• User cannot easily pick whichever authentication method he sees fit but is presented with one option at the time. Switching to e.g. password login is unintuitive and unreliable at best. (Pressing esc sometimes switches, sometimes restarts the fprintd and sometimes seems to do nothing.) Preferably all the authentication methods would be active at once.

On the other hand, cinnamon-screensaver seems to handle it somewhat better. Maybe it would be possible to share this piece of logic between both tools?

The issue names no source files or tests; start by reproducing the fprintd and PAM login flow in slick-greeter and compare it with the referenced cinnamon-screensaver commit. Done means authentication messages and controls do not overflow, successful authentication proceeds without extra input, and users can reliably choose between available methods.