Investigate how MacDown handles key events. The issue likely relates to the NSTextView subclass's keyDown: method not properly passing function keys like Home, End, PageUp, PageDown to the responder chain. Look at the MacDown source code, specifically the editor view files, to see how keyboard events are intercepted. Check if there is a custom key handler that blocks these keys or if the key equivalents are missing. Compare with system level key bindings and consider adding support for external keyboards with different key codes.
Home, End, etc on external keyboard do not work · MacDownApp/macdown#1020 | Good First Issue