bug#28955: 27.0.50; mouse event translation loses parameters

[Sam Steingold <sds@gnu.org>]

In GNU Emacs 27.0.50 (build 3, x86_64-apple-darwin16.7.0, NS appkit-1504.83 Version 10.12.6 (Build 16G29))
 of 2017-10-16 built on Clr-Sam.local
Repository revision: eed3a3d9e95d2c5346a23c9d92ca4e5848330183
Windowing system distributor 'Apple', version 10.3.1504
Configured using:
 'configure --with-mailutils --with-ns
 PKG_CONFIG_PATH=/usr/local/opt/libxml2/lib/pkgconfig:/usr/local/opt/imagemagick/lib/pkgconfig/
 --without-makeinfo'

Configured features:
JPEG NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS

Important settings:
  value of $LC_ALL: en_US.UTF-8
  value of $LANG: C
  locale-coding-system: utf-8-unix

Some platforms lack second mouse button (most laptops), and some lack
both 2nd and 3rd (mac).
The almighty internet (https://emacs.stackexchange.com/q/20946/795 &
https://superuser.com/q/364575/98857) suggests

(define-key key-translation-map (kbd "<s-mouse-1>") (kbd "<mouse-2>"))

Alas, when I actually do command-mouse-1, I get an error:

--8<---------------cut here---------------start------------->8---
Debugger entered--Lisp error: (error "mouse-yank-primary must be bound to an event with parameters")
  call-interactively(mouse-yank-primary nil nil)
  command-execute(mouse-yank-primary)
--8<---------------cut here---------------end--------------->8---

Charles writes:

When you add the key translation and type s-mouse-1, the mouse click
event ends up losing its location information.

For example, the following:

(s-mouse-1 (#<window 3 on *scratch*> 227 (116 . 92) 143676325 nil 227 (16 . 5) nil (116 . 12) (7 . 16)))

gets turned into just:

mouse-2

which leads to the error about the missing parameters.

It happens in this part of keyboard.c:

--8<---------------cut here---------------start------------->8---
      /* Does mock_input indicate that we are re-reading a key sequence?  */
      if (t < mock_input)
	{
	  key = keybuf[t];
	  add_command_key (key);
	  if (current_kboard->immediate_echo)
	    {
	      /* Set immediate_echo to false so as to force echo_now to
		 redisplay (it will set immediate_echo right back to true).  */
	      current_kboard->immediate_echo = false;
	      echo_now ();
	    }
	}
--8<---------------cut here---------------end--------------->8---

For further details please see
https://lists.gnu.org/archive/html/emacs-devel/2017-10/msg00667.html
https://lists.gnu.org/archive/html/emacs-devel/2017-10/msg00678.html

-- 
Sam Steingold (http://sds.podval.org/) on darwin Ns 10.3.1504
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