Problem: An already open "more" window is the most convenient place
to route messages to with `cfg.msg.pos == 'cmd'`. Instead we
route to the cmdline (unless that message also exceeds
'cmdheight').
Solution: Route to "more" window while it is open and scroll to bottom
to show the newest message. This is more convenient and more
efficient due to not writing to the target buffer first (which
is done to calculate the height of the message).
(Ensure message highlights are deleted when text is replaced.)
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