narrow non break space is 202f and typing 202f and then alt+x works fine
1f408 is cat and typing that and alt+x again works fine
so not sure what your issue is. Yeah it doesnt work in all text fields, but that would be true regardless what shortcut is used.
I dont know what ctrl+shift+u on your linux box is doing, it doesnt do anything on mine. I am guessing it is triggerring a unicde char picker via IME. IME may or may not work with Qt depends on plugin versions/ABI compatibility, display backend, etc.
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