There is a key assigned to ‌‌Zero Width Non-Joiner character, U+200C, but it is not printed in KDE-based (or probably qt-based) apps. For example, the key does not work in kate but it works in gedit. Is there a way to solve this problem?

There is a similar problem with some other space characters. To reproduce the problem, switch, for example, to Arabic keyboard layout, the press Shift+Space both in kate and gedit to to see the difference.

The problem is that the character is not entered at all in kde-based apps. Nothing is typed in kate as if no key is pressed. The problem seems to be clear. However, if I type the character in gedit and then copy and paste it in kate, I will have the character in kate. So the problem is that nothing is typed in KDE-based apps.

This is a bug in KDE's text editor document. I posted a workaround (details are in Persian) for Ubuntu users of KDE here:

https://soroush.github.io/blog/kates-zwnj-bug/