bug#37667: 27.0.50; Tab Bar display problems with more than 5 tabs

[Juri Linkov]

> But that won't work with variable-pitch fonts and with different
> images for the add and close buttons, would it?  Also, doesn't
> tab-line-format allow faces (which could change font size) and dynamic
> elements via the likes of :eval, like mode-line does?  these would
> prevent you from finding where to cut the content, no?
>
> But if this works, by all means go ahead and install something like
> that.

There are two related but separate features: scrolling and auto-scrolling.

Scrolling is implemented now and installed.  It provides UI with buttons,
commands, and mouse-wheeling.

Auto-scrolling could be implemented as well for user convenience: like when
auto-hscroll-mode is enabled, the tab-line could do the same - when due
to the long row of tabs, the current tab gets pushed over the right edge
outside of the screen, then the tab-line could be automatically scrolled
horizontally to make the current tab visible.

This could be implemented by setting the right value to the new window
parameter tab-line-hscroll, and it will bring the current tab back to the view.

The main problem in implementing auto-scrolling is how to detect the
situation when the current tab is not visible.  Maybe by implementing
a function like pos-visible-in-window-p, but named tab-visible-in-tab-line?