bug#27517: 26.0.50; 25.2.2; Cannot add "help-echo" property to buffer regions.

[Eli Zaretskii]

> From: Vitalie Spinu <spinuvit@gmail.com>
> Date: Wed, 28 Jun 2017 14:35:07 +0200
> 
>  (put-text-property (point-min) 100 'help-echo "my-mouse-over" 
> (current-buffer))
> 
> No "help-ehco" property is added. Any other property (including special
> properties) can be added.

But only in lisp-mode, right?

This happens because lisp-mode.el wants itself to manage the help-echo
property:

  (defun lisp-mode-variables (&optional lisp-syntax keywords-case-insensitive
                                        elisp)
  ...
    (setq font-lock-defaults
          `(,(if elisp '(lisp-el-font-lock-keywords
                         lisp-el-font-lock-keywords-1
                         lisp-el-font-lock-keywords-2)
               '(lisp-cl-font-lock-keywords
                 lisp-cl-font-lock-keywords-1
                 lisp-cl-font-lock-keywords-2))
            nil ,keywords-case-insensitive nil nil
            (font-lock-mark-block-function . mark-defun)
            (font-lock-extra-managed-props help-echo)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When a property appears in font-lock-extra-managed-props, font-lock
removes that property when it's about to re-fontify some region, see
font-lock-default-unfontify-region.

CC'ing Stefan, who made that change.