Function picker_key 

pub fn picker_key(
    picker: &mut PickerState,
    multi: bool,
    event: &KeyEvent,
) -> PickerKey

What event does in an open picker: type to narrow, ↑↓ move, Space chooses (or toggles in a list of several), Enter chooses, Esc closes the picker only. A space never reaches the narrowing filter: it would match nothing and blank the list under the key that opened it.