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

[Juri Linkov <juri@linkov.net>]

> However, I hoped we could have at least part of what you did in Emacs
> 27, and not lose all of it.  We could install a function like
> 'minimess-minibuffer-message', and make one of the subroutines of
> 'message' call it under the right conditions.

The most low-level functions where it could be called from
are 'set_message' and 'clear_message'.

> Then the message which
> 'minimess-minibuffer-message' displays could be removed by a timer if
> the new timeout defcustom is a number, and if that defcustom is not a
> number (which will be the default), the message stays and sit-for is
> not called.

This is almost how 'minimess-minibuffer-message' was implemented,
with one difference: defcustom is a number by default.
Why shouldn't it be a number by default?  How the user would be able
to remove an old message when the function doesn't call sit-for?

> Do you see any danger with the above?  If not, can I persuade you do
> install such a change?

I'm not sure if now is the right time to implement this feature.
But if implementation would be straightforward and if you see no problems
then why not.

>> (defun minimess-message (orig-fun format-string &rest args)
>>   (if (and
>>        ;; When `inhibit-message' is non-nil, the intention was to just
>>        ;; log the message to the *Messages* buffer using `message'.
>>        (null inhibit-message)
>>        (window-live-p (active-minibuffer-window))
>>        (window-live-p (old-selected-window))
>>        (bufferp (window-buffer (old-selected-window)))
>>        (minibufferp (window-buffer (old-selected-window))))
>
> Btw, can you explain why every part of this condition is needed?  IOW,
> why isn't just the below enough?
>
>    (window-live-p (active-minibuffer-window))
>
> (I do understand the reason for the test of inhibit-message).
>
> Maybe the other conditions need a comment to explain them?

Unfortunately, I forgot why they were added, i.e. during testing
I added them one by one when noticed that some cases don't work.
Now I'll try to reproduce these cases by removing conditions
and checking which part doesn't work.