Same issue, haven't found any answer yet.

after a system update, it works, closed

figure it out    public func textView(_ textView: NSTextView, shouldChangeTextIn affectedCharRange: NSRange, replacementString: String?) -> Bool { if replacementString == "K" { return false } return true }

having the same issue on swift 6 macos 15.1 xcode 16.0 beta4