Hand focus to the reason input while it is open.
on_blur deliberately stops re-trapping focus during reason mode so the
Input can hold it, which leaves the reverse direction unhandled: a
click on the menu body focuses the menu and strands an open reason field,
where quick keys read as menu commands instead of text. Bouncing focus
back keeps "field open" and "field focused" the same state.
Cannot ping-pong: this focuses the Input, whose gain of focus blurs the
menu, and on_blur above returns early during reason mode. Nor does it
trap the user - _exit_reason_input_mode clears the flag before moving
focus, so a cancelled field is not re-entered.
