bug#38457: 27.0.50; dabbrev-expand regression due to message change

[Juri Linkov]

>> Such variable already exists.  It's called message-in-echo-area.
>> You can enable it in the release branch if you want.
>> But then please reopen bug#34614, bug#19064, bug#17272, bug#446.
>
> Sorry, I don't understand the proposal.  How will this variable help
> if we leave the current code in 'message' as it is?  And what do you
> mean by "enabling" message-in-echo-area?

When message-in-echo-area is non-nil, then 'message' has exactly
the same behavior as before, as in the previous versions.

I propose to use timers in minibuffer-message conditionally
only when message-in-echo-area is nil.  Then the behavior
of minibuffer-message will be exactly the same as before too
when message-in-echo-area is non-nil.

If you want to set message-in-echo-area to t in the release branch,
that's fine.

Then everyone who doesn't fear timers can customize it to nil.