Sometimes when I come back to my computer and it has locked - in particular, after a suspend-resume - the focus of the lock screen is not on the password field. I only notice that when I start typing. It happened at least two times that I accidentally changed my keyboard layout rather than entering my password, and it took me a while to figure out how that could happen... Reproducible: Sometimes I couldn't figure out which package the lock screen belongs to.
It seems the part about changing the keyboard layout is actually not caused by me accidentally using that widget on the lock screen. I now have only one keyboard layout configured in systemsettings, so there is no such widget. Still, every now and then, when the machine comes back from stand-by, the keyboard layout changed to something else.
I have the same issue (loosing password field focus). Generally it is probably caused by some application (or even plasmashell) requiring focus. I can reproduce this always like this: 1) Start e.g. SQL Developer - it starts launching with splashcreen 2) Lock screen 3) When SQL Developer is fully started it's window go always on the top of others -> this causes losing the focus of the password field in locked screen Another way to reproduce it: 1) Restart plasmashell 2) Lock screen 3) Plug in usb flash disk -> pasword field focus is lost ...this works only for the first time a flash disk is plugged in (that's why the restarting plasmashell before is necessary to reproduce this bug)
Please try again with Plasma 5.5. At least the problem described in comment #2 of other applications breaking the focus on the lock screen is fixed. If you are still able to reproduce with Plasma 5.5 please reopen.
Indeed, I cannot reproduce this issue anymore with Plasma 5.5.0, KF 5.16. Thanks for the fix! :-)