bug#52293: 29.0.50; [PATCH] Prevent further cases of duplicated separators in context menus

[Juri Linkov <juri@linkov.net>]

> On 12/5/2021 9:59 AM, Juri Linkov wrote:
>>>   (defun help-mode-context-menu (menu click)
>>>     "Populate MENU with Help mode commands at CLICK."
>>> -  (define-key menu [help-mode-separator] menu-bar-separator)
>>> +  (define-key-after menu [help-mode-separator] menu-bar-separator
>>> +    'top-separator)
>> Now I realized that it's possible to do the same without 'top-separator':
>>    (define-key-after menu [help-mode-separator] menu-bar-separator
>>      "Context Menu")
>> Or when the title string is defined as a variable:
>>    (defvar context-menu-title "Context Menu")
>>    (define-key-after menu [help-mode-separator] menu-bar-separator
>>      context-menu-title)
>> But maybe 'top-separator' still could be used for clarity?
>> Or it increases complexity?
>
> Hmm, that might work. One downside is that I think it makes it harder for
> context menu functions to change the menu's title/prompt to something
> else. Of course, if we used `context-menu-title' as the anchor like your
> example above, it should still be possible to update the menu title via
> `context-menu-filter-function'. That would be trickier to use though, at
> least in the situations I have in mind.

I agree that relying on the constant value of the menu title
would be too unreliable.

> For example, I added a very limited context menu that uses the menu title
> in my config under Emacs 27 for org-mode links: I can right-click and it
> shows a context menu with the URL as the title and menu items for different
> ways to open the URL (in Firefox, Firefox Private Browsing, or EWW). It's
> nice to be able to see the URL since that can influence which item
> I choose.

Good example.  Another example is flyspell that shows a misspelled word
in the menu title.

> Updating this part of my config for Emacs 28 was actually what prompted me
> to start looking into `context-menu-mode' in more detail. It would be
> easier to implement this if context menu functions didn't rely on the
> context menu title having a particular value.

I think the remaining problem we have to solve is to try to
prevent such a situation when the user accidentally removes
`context-menu-top-separator' from `context-menu-functions'.
To not break the menu in this case, maybe we need to ensure
that the menu always contains `top-separator' as well as
`middle-separator' even when `context-menu-functions'
doesn't contain a function that adds them?  I.e. just to check
if these separators are missing, and add them at the top.