* src/keyboard.c (syms_of_keyboard):
<multiple_terminals_merge_keyboards>: New variable.
(read_event_from_main_queue): Mention it in message.
* lisp/cus-start.el: Make it customizable.
* src/callint.c (Ffuncall_interactively):
* src/keyboard.c (Frecursive_edit):
* src/minibuf.c (read_minibuf): When its value is true, don't
enter single-kboard mode (bug#79892).
* admin/notes/multi-tty:
* etc/NEWS: Document it.
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