bug#42484: 26.1: org-mode should display value of links in mini-buffer

[Juri Linkov]

> Still, I would like to continue to promote my solution, because it's
> much simpler and is instantaneous upon key-press. It might also be more
> efficient: The help-at-pt solution runs code in all buffers, let's say
> every 0.1 seconds, all the time; my solution only runs in the selected
> mode(s) buffers but after every key-press, which for an 'average'
> touch-typist taking a speed test would be 0.3 seconds.

I agree.  Overhead of needlessly running the global timer is what concerns
me too.  But using an idle timer by help-at-pt is not that bad either.
It runs code only after the last key-press in a sequence of many key-presses.
So with idle timer in help-at-pt and the default delay, code runs less often
than by using post-command-hook.  Here are a brief comparison of
advantages and disadvantages of these two approaches:

1. help-at-pt idle timer

Pros:
1.1. runs code once a sequence of key-presses is finished,
     and 1 second has passed after the last key-press,
     where 1 second is the default value of help-at-pt-timer-delay.
     Customizing it to 0.1 removes this advantage because on average
     there is more time between key-presses than 0.1 seconds.

Cons:
1.1. With a bigger value of help-at-pt-timer-delay (by default, 1 second)
     that helps code to run less often (not after every key-press),
     the effect of the primary goal of this feature to display
     the help-echo string is not instantaneous;
1.2. the timer runs globally in all modes (this could be mitigated
     by checking major mode in the timer function).

2. post-command-hook

Pros:
1.1. can be activated locally only in org-mode buffers;
1.2. display of the help-echo string is instantaneous.

Cons:
1.1. runs code after every key-press.

So your approach has more advantages.  The only problem with your code
is that it displays the garbled mojibake on URLs with Unicode symbols,
that need to be decoded to UTF-8 with:

  (message "%s" (decode-coding-string (url-unhex-string msg) 'utf-8))

Also not to step on other more important minibuffer echo-area messages,
help-at-pt-maybe-display has better handling with:

       (or (not (current-message))
           (string= (current-message) "Quit"))