bug#69993: Wrap window buffers while cycling

[Juri Linkov <juri@linkov.net>]

> Suppose I'm at the first element of a tab list.  With wrapping/cycling
> 'switch-to-prev-buffer' will select some buffer and that buffer will be
> shown as current in the tab list.

The selected buffer will be the first element of a tab list, not the last
as wrapping should do.  So currently 'switch-to-prev-buffer' doesn't do
wrapping.  Before 'switch-to-prev-buffer' the leftmost tab is selected,
and after 'switch-to-prev-buffer' the leftmost tab will remain selected.
Whereas the proper wrapping should select the rightmost tab/buffer.

> With restricting it will be the last element of the tab list, without
> restricting it may be a buffer that's new on the tab list, probably
> shown at its beginning.

When a buffer not previously displayed in the window appears at the
beginning, this is not wrapping.  Also when all available buffers were
already displayed in the window, and so restriction has no effect,
and an already displayed buffer appears at the beginning, this is
not wrapping too.

> Since 'tab-line-switch-to-prev-tab' restricts by default, it will
> always select the last buffer on the tab list.  What am I missing?

Unfortunately 'tab-line-switch-to-prev-tab' doesn't restrict by default.
This is a big problem for users that needs to be fixed.  Therefore
the wrapping option was proposed either for 'switch-to-prev-buffer'
or for 'tab-line-switch-to-prev-tab'.

>>> 'tab-line-switch-cycling' could then be used to override the more
>>> general 'switch-to-prev-buffer-wrap' when calling
>>> 'tab-line-switch-to-prev-tab'.  Or we could alias
>>> 'tab-line-switch-cycling' to 'switch-to-prev-buffer-wrap'.  And we could
>>> use the term 'switch-to-prev-buffer-cycle' instead.
>>
>> I think "cycle" is a wrong name.  Cycling is using
>> 'switch-to-prev-buffer'.  But wrapping is going
>> from the beginning to the end of the tab-line.
>
> In my understanding, the elements of the tab line are a visual
> representation of a "window-local" subset of all buffers.  This subset
> is ordered in a way that does not necessarily match the order used by
> 'switch-to-prev-buffer' - when C-x b-ing to a buffer already on the tab
> line, the tab line order does not change while the order of buffers used
> by C-x <left> and C-x <right> may change.  Other than that, cycling and
> wrapping behave identically wrt their respective sets.

By default the tab-line uses the order of window prev/next buffers.
So maybe it would be easier to keep the order in tab-line.el.

>> Mixing wrapping/cycling and restricting is unavoidable:
>> you can see how the previous patch reorders prev/next buffers
>> to make the order of tabs stay the same after wrapping.
>
> The order of tabs is one thing, that of buffers on the buffer list
> another and that of buffers to be visited by 'switch-to-prev-buffer' and
> 'switch-to-next-buffer' a third one.

Currently the first thing is the same as the third thing,
and this works fine.