If you refer to the bumped post regarding field validation.
Unfortunately your linked source-file only refers to server-side validation (which works perfectly btw), the problem is clientside validation (where the field gets a red border, or warns before posting to the server that something is wrong with the input).
In the textInputsource-file (which I mostly use for reference) there are a bunch of imports at the top of the file. Those also contain the clientside-validation logic for the field. The problem is that I don’t use a js-module environment, meaning I cannot use the import mechanics directly myself.
I thought I was able to reuse/access these mechanics when you extend another field (also client-side), but that doesn’t seem to do anything unfortunately. I can try to reinvent the wheel, but I was hoping I could use the mechanics Kirby already provides.