Function send_key 

pub unsafe fn send_key(
    window: *mut c_void,
    key_code: u16,
    characters: &str,
    flags: u64,
)

Send a key down and up straight to window.

post_key puts the event on the application’s queue, which is what a tracking menu reads. This hands it to one window with sendEvent:, the same route send_mouse takes — which is what a check driving a focused view wants, since it does not depend on which window the application thinks is key beyond that window being able to accept it at all.

flags are NSEvent.ModifierFlags, so 1 << 20 is Command and 1 << 17 Shift. They are what SwiftUI turns into the EventModifiers a KeyPress carries.

Safety

window must be a live NSWindow, and this must run on the main thread.