The full correct solution is as follows:
In this directory you will find several files:
michael@Kubuntu-VM:/etc/sddm.conf.d$ ls -la
drwxr-xr-x 2 root root 4096 Oct 8 11:08 .
drwxr-xr-x 186 root root 12288 Oct 8 11:05 ..
-rw-r--r-- 1 root root 167 Oct 8 11:08 kde_settings.conf
Open up the kde_settings.conf in your editor of choice (as superuser, because it is owned by root):
sudo vim kde_settings.conf
You should see something like the following:
Now, right under whatever is the last entry in your
General group, add the setting
InputMethod=. You can optionally add the comments shown in the example below above this line for future reference. I included a link to this answer, explaining the line's necessity, for additional info. I did this in case you encounter this file and its change at some time in the distant future, probably while upgrading your Linux distro or some package which wants to reset this file, by which point you will have likely forgotten all of this and the reason why you made this change - it happens to all of us:
# This disables the on-screen keyboard from popping up when
# Username and Password are being entered at the Login screen.
# For additional info, see: https://askubuntu.com/a/1368158/4197
Log out and test by trying to reenter your username and password, hopefully without that annoying gratuitous on-screen keyboard popping up and obscuring the entire Login screen, including the field you are trying to populate - an excellent example of horrible UI design.
Shout, "Finally!", and get on with whatever task you were trying to do before encountering this frustration and winding up here in askubuntu (e.g., updating your SDDM Login theme and getting hijacked by this keyboard gremlin).
For those still here and desiring to understand the "why of it all," enter the following command:
/usr/bin/sddm --example-config | grep Input
# Input method module
The above default for the
InputMethod, which is probably this way for phone/tablet support, is the reason you are here.