SUMMARY
If you enter a password on the lock screen, then press the eye icon to make it visible, then press Return or click the arrow, the password should be immediately masked (i.e. turned into circles) before anything else. See Additional Info for why.

STEPS TO REPRODUCE
1. Lock your screen
2. Type your password in
3. Press the eye button to make the password visible
4. Press Return

OBSERVED RESULT
The password is still visible for a short while (see Additional Info).

EXPECTED RESULT
The password is immediately masked.

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20240808
KDE Plasma Version: 6.1.4
KDE Frameworks Version: 6.4.0
Qt Version: 6.7.2
Kernel Version: 6.10.3-1-default (64-bit)
Graphics Platform: X11

ADDITIONAL INFORMATION
I have two use cases for this:

1, I was using an old laptop with not a lot of RAM, which spends a lot of time swapping (I realise this is not optimal, but anyway). I typed in my password, pressed the eye to check it (as I typed it wrong the first time), then pressed Return. Because of the low performance/high swapping of the machine, the correct password was left visible for three or four seconds while the PC was churning away, before the screen unlocked.

2, Even on a fast performance machine, if you unmask the password and still manage to type the password in incorrectly, the incorrect password is left visible for about three seconds before it disappears, even though it is greyed out immediately.

As I say, in my opinion, in both these cases the password should be masked immediately you press Return/arrow (in the same way it's greyed out immediately), before any further processing as to whether the password is correct or not.

Presumably there are other situations, not just slow machines, that may cause slow checking of passwords, such as when using LDAP on a slow network, where again you would not want the password to continue to be visible for so long. Thanks for reading!