bug#33320: 27.0.50; `describe-key' fails to display help for C-mouse-3

[Eli Zaretskii <eliz@gnu.org>]

> From: "N. Jackson" <nljlistbox2@gmail.com>
> Date: Thu, 08 Nov 2018 16:49:24 -0500
> 
> 0. $ src/emacs -Q
> 1. C-h k C-mouse-3
> 
> Expected behaviour:
> 
> A help buffer describes whatever command it is that is bound to
> C-mouse-3.

I don't think we can do that with all toolkits, because once Emacs
pops up the menu, the toolkit has control.

In any case, C-mouse-3 is a kind of prefix key, and we never display
help for prefix keys, we always wait for the user to type the complete
key sequence, which in this case includes menu selection.

> Observed behaviour:
> 
> A context menu pops up and in the echo area, "Describe the following
> key, mouse click, or menu item: C-down-mouse-3-" is displayed. But then
> one is stuck:
> 
> - If one hits ESC, the context menu disappears, "Quit" appears in the
>   echo area, and no help buffer is displayed.
> 
> - If one hits RET to accept the value in the prompt in the echo area,
>   annoyingly nothing happens at all, and one stays in the same stuck
>   state.
> 
> - If one clicks in the echo area to try to change "C-down-mouse-3-" to
>   "C-down-mouse-3", or just to be sure the RET goes to the minibuffer
>   not to the context menu, "Quit" appears in the echo area, and no help
>   buffer is displayed.

You need to select one of the menu items by clicking on it, then you
will see the help for that item.