bug#41200: Displaying a tooltip with x-show-tip gets very slow as more faces are defined

[Juri Linkov]

>>> +  :group 'basic-faces)
>>> +
>>> +  :group 'basic-faces)
>>>
>>> -  :group 'tab-bar-faces)
>>> -
>>> -  :group 'tab-line-faces)
>> 
>> Could these faces belong to both groups?
>> 
>>   :group 'basic-faces
>>   :group 'tab-bar-faces
>
> Yup, but that won't work well if the face is customized before the
> group is defined, right?  Maybe the trick would be to add the group to
> the face when tab-bar.el and tab-line.el are loaded?

Actually since tab-bar.el is pre-loaded this problem exists only for
tab-line.el.

Then adding something like

  (nconc (get 'tab-line-faces 'custom-group) '((tab-line custom-face) ...))

to tab-line.el will add faces when tab-line.el are loaded.