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

[Juri Linkov]

>> The original problem was, AFAIU, that various minibuffer prompts
>> become obscured by echo-area display of messages.  So one possible
>> solution is to modify the subroutines of 'message', e.g.,
>> set_message_1, to detect the conditions of the minibuffer being
>> active, and insert the contents of the minibuffer into the echo-area
>> buffer before the message text.  Does anyone see problems with this?
>
> Juri, could you please share your opinion about this proposal?  you
> worked on changes related to this issues a lot lately, so you probably
> know about this more than anyone else.  Do you see any problem with
> the above?  If workable, I think its advantage is that it minimizes
> behavior changes, while preserving the main traits of the solution: to
> avoid obscuring minibuffer prompts by asynchronous messages.

The problem is not in implementation.  Of course, it's possible
to display the minibuffer's contents with an appended message
in the echo-area like Isearch does.

The problem is in usability: it would be very annoying if the message
displayed at the end of the minibuffer's contents would not vanish
after some time.  minibuffer-message removes the message
after 2 sec by default.

If someone wants the message to hang out indefinitely in the minibuffer,
this is possible, minibuffer-message-timeout is configurable:

  minibuffer-message-timeout is a variable defined in ‘C source code’.
  Its value is 2
  Documentation:
  How long to display an echo-area message when the minibuffer is active.
  If the value is a number, it should be specified in seconds.
  If the value is not a number, such messages never time out.

But this means that your proposed implementation still should use timers
to remove the echo-area with the appended message after the amount of time
specified by minibuffer-message-timeout is passed (when its value is a number).