bug#40774: Error messages shouldn't be hidden when the user is idle

[Juri Linkov]

>> +** The return value of 'clear-message-function' is not ignored anymore.
>> +If the function returns t, then the message is not cleared,
>> +with the assumption that the function cleared it itself.
>
> I could perhaps agree to this if the special new behavior was the
> result of a very special return value, and only that value.  Having
> the new behavior kick in for t is out of the question for the release
> branch, as it is highly likely to trip unsuspecting Lisp programs.

What a special value would you prefer?  Maybe, a symbol 'no'?

> Btw, what does the change of the order between the call of
> clear-message-function and setting echo_area_buffer[0] to nil mean,
> compatibility-wise? won't it also produce different results, even if
> the return value is nil?

When the return value is nil, it will still clear the echo area.

> More generally, I fear that we are trying very hard to tweak a
> particular infrastructure for a job for which it was hardly meant.

This is the most simple and thus reliable solution.

> IOW, shouldn't we provide some completely different optional feature
> for this use case?  Like a special buffer that pops up or a special
> frame?  Echo-area is not suited for showing large chunks of text, and
> my gut feeling is that we will bump into problems on this path.  E.g.,
> what happens when there are enough accumulated messages that they can
> no longer be shown with the maximum allowed height of the mini-window?

This is exactly what functions bound to clear-message-function intended to do.