bug#29478: 26.0.90; `C-h k' followed by mouse clicks no longer shows down event

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 28 Nov 2017 20:33:00 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: 29478@debbugs.gnu.org
> 
> > > > The down mouse-button event is no longer listed along with the up
> > > > event when you use `C-h k' with a click event.  Dunno whether this
> > was
> > > > by design or is an oversight.
> > 
> > I think if you press and hold the mouse button for (default) half a
> > second, when you finally release it, C-h k will report the down mouse
> > event.  At least this is how GPM behaves for me on a Linux tty.
> 
> Ah yes, thanks.  I see that now on MS Windows too.
> 
> > This seems to make sense, because anything bound to a down mouse
> > event is going to be something like a drag event, where the mouse
> > button is held for an extended period of time.
> 
> Sorry, no, I don't buy that.  I mean it's probably true that
> most such bindings involve dragging.  But (1) a user who wants
> to check what `mouse-N' does won't necessarily use it the same
> way when checking with `C-h k' as when using it for its real
> purpose.  And (2) the problem with this is that it is not
> easily discoverable.
> 
> I've used Emacs for quite a while.  OK, I expected both up and
> down to be documented.  But even I would likely never have
> thought to hold the button pressed for a long time before
> releasing, just to see what `C-h k' had to tell me.

Alan, can we arrange for the doc string of mouse-N clicks (with or
without all the possible modifiers) to say something like

  For documentation of the corresponding mouse-down event, click and
  hold the mouse button longer than %s sec.

(where %s gets replaced by double-click-time)?