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

[Alan Mackenzie]

Hello, Eli.

On Sat, Dec 09, 2017 at 18:13:40 +0200, Eli Zaretskii wrote:
> I think I found one more problem with Help functions on mouse clicks.
> Try this in "emacs -Q":

>   C-h c S-mouse-1
>   C-h k S-mouse-1

> In Emacs 25.2 these produce the description of mouse-appearance-menu,
> as expected, saying it's the binding of S-down-mouse-1.  Emacs
> 26.0.90 does the same for "C-h c", but claims S-mouse-1 is undefined
> if you invoke "C-h k".  The current emacs-26 branch claims S-mouse-1
> is undefined for both "C-h c" and "C-h k".

> I think this means that we have similar problems with any explicit
> bindings to S-down-mouse-N clicks.

> This is a regression that I think we must fix before releasing Emacs
> 26.1.

OK, I'm looking at this now.  It's not as easy as I was hoping.

One phenomenon is puzzling me - it feels like a bug:

(i) emacs-26 -Q
(ii) M-: (read-key-sequence "prompt: ") <CR>
(iii) <shift>-mouse-1

What happens now is the key sequence is displayed, a S-down-mouse-1
event, but immediately overwritten in the message area by 

    <S-mouse-1> is undefined

.  Why?  I merely asked for the key sequence.  I didn't ask for it to be
looked up in any key maps.  What is doing the translation from a
shift-down-mouse event to a S-click event?  What is looking up this
down-mouse event in a key map?

-- 
Alan Mackenzie (Nuremberg, Germany).