bug#74087: Bug patch: invoke tab-bar-tab-post-open-functions during tabs initialization

[Juri Linkov <juri@linkov.net>]

>> I'm trying to align all tab creation behavior with
>> tab-bar-tab-post-open-functions. Even the first tab should be
>> considered "opened" by the system and call people's functions on the
>> hook. That tab, if you close it, does call the close hooks correctly
>> and that's aligned, at least.

Calling the close hooks currently doesn't look correct:

      (run-hook-with-args 'tab-bar-tab-pre-close-functions
                          (nth close-index tabs)
                          last-tab-p)
      (if last-tab-p
          (pcase tab-bar-close-last-tab-choice
            ('nil
             (user-error "Attempt to delete the sole tab in a frame"))

where first it makes an attempt to close the tab, calls the hook,
then fails with the user-error.  I think the hook should be called
when the tab is really closed.

>> If the open hook doesn't get called, then
>> those tabs have to be treated in a special way and that creates bugs
>> and burden. Not sure how relevant it is what I do in my custom hook
>> that should get called for the implied opened first frames but doesn't.
>
> If you're really curious, one use case is auto-assigning a tab group. When
> one makes a frame, the tab bar defaults to showing the window from which it
> was created and any custom tab-bar symbols attached to the tab have to be
> recreated. Another use case is optionally keeping tabs "pinned" to their
> user-specified positions and the post-open hook can help track the first
> and subsequent positions. I'll surely add more over time. tab-bar is great
> and fun to use and enhance with cool productivity tools.

Thanks, this is an interesting case.  Indeed, currently
there is an inconsistency.  For example, start `emacs -Q`.
Even if there is no tab-bar initially, we can say that
the first tab is already created.  Later, after creating
the second tab with 'C-x t 2', we have two tabs where
the post-open hook was called only in the second tab.

A possible solution would be to detect such situation
in 'tab-bar-new-tab-to' with the same code that
enables the tab-bar initially with:

    (when tab-bar-show
      (if (not tab-bar-mode)
          ;; Turn on `tab-bar-mode' since a tab was created.
          ;; Note: this also updates `tab-bar-lines'.
          (tab-bar-mode 1)
        (tab-bar--update-tab-bar-lines)))

then run the hook in the first tab as well.