bug#55070: 28.1; desktop-load doesn't work in -nw (non-gui) emacs

[Juri Linkov <juri@linkov.net>]

>> Saving the desktop in a GUI session and restoring in a TTY session
>> adds an empty line between the top buffer and the tab bar when
>> tab-bar-mode was enabled before saving.
>
> This is a subtle bug in how we compute the tab-bar-lines frame
> parameter.  Observe:
>
>  emacs -Q
>  M-x tab-bar-mode RET
>  M-: (frame-parameter nil 'tab-bar-lines) => 2
>
> Surprise!
>
> The problem is that the tab-bar line is slightly higher than the
> canonical line height of the frame, so when we compute the height in
> lines, we get 1 more.
>
> I think this means we cannot save and restore tab-bar-mode by relying
> on the tab-bar-lines frame parameter; the solution should be in a
> special support for desktop.el in tab-bar.el.  For example, remove the
> tab-bar-line frame parameter when saving the desktop, and instead save
> the tab-bar-mode state; then restoring the desktop would turn on
> tab-bar-mode in the restored session, and recreate the tab bar of the
> desired height.

Agreed.  Although currently I have no idea how to do this without adding
direct calls of tab-bar.el functions in desktop.el.

> I hope you can develop such a solution, so that tab bars could be
> meaningfully restored on TTY frames.

Such a solution is also needed for GUI frames as well, because
when tab-bar-mode is not enabled explicitly, then after restoring
the desktop with tab-bar-line frame parameters, tab buttons are too ugly.
The graphical versions of these buttons are created only when
tab-bar-mode is enabled on a GUI frame.  So desktop.el should
enable tab-bar-mode somehow.

> Btw, what about tab-bar-show -- should it be saved as well? at least
> if its value is not the default?

tab-bar-lines are updated according to the value of tab-bar-show
in tab-bar--update-tab-bar-lines (called from tab-bar-mode).
So maybe desktop.el should call tab-bar--update-tab-bar-lines too.

But this raises another question: when the user changes the value
of tab-bar-show, should desktop.el show the tab bar exactly as saved,
or should it update the tab bar according to the new value of tab-bar-show
immediately after restoring the desktop?