For certain languages, the ZWNJ is an important character in text shaping. For example, the following text in Farsi has an ZWNJ:

نیم‌فاصله

For those locales, the U+200C character is often found bound to a key in the keyboard. However, when pressing it, nothing is inserted in text fields in Qt applications.

To test:

1. Assign it to the Shift+Space combination. Run:‌xmodmap -e 'keycode 65 = space U200C
2. Open a Qt application and select a text field
3. Press Shift+Space

Nothing is inserted. Since this is a zero-width character, this can usually be seen by noticing if the text editor (like Qt Creator)‌ adds a star indicating the file is modified:‌ it doesn't.

qttools/src/qev shows the event is properly decoded:

QKeyEvent(KeyPress, Key_Shift, ShiftModifier) 
QKeyEvent(ShortcutOverride, 8204, ShiftModifier, text="\u200C") 
QKeyEvent(KeyPress, 8204, ShiftModifier, text="\u200C") 
QKeyEvent(KeyRelease, 8204, ShiftModifier, text="\u200C") 
QKeyEvent(KeyRelease, Key_Shift)