bug#62237: 28.1 or higher: 24-bit true color breaks colours in Emacsen built without X under GNU Screen

[Sebastian Tennant <sdt@sebyte.me>]

Quoth Eli Zaretskii <eliz@gnu.org>
on Sat, 18 Mar 2023 11:09:10 +0200:
>>>> […] machines you are able to do a rlogin and still keep the
>>>> correct termcap/terminfo entry.  The terminal name is put in the
>>>> $TERM variable of all new windows.  Screen also sets the $TERMCAP
>>>> variable reflecting the capabilities of the virtual terminal
>>>> emulated. Notice that, however, on machines using the terminfo
>>>> database this variable has no effect.  Furthermore, the variable
>>>> $WINDOW is set to the window number of each window.
>>>>
>>> This seems to tell how 'screen' figures out the terminal name, not
>>> how it sets TERM.  I asked who and why sets TERM to
>>> screen.SOMETHING.
>>>
>> screen does: "The terminal name is put in the $TERM variable of all
>> new windows."
>>
> So how did Emacs ever succeed to work inside screen, then?  AFAIK,
> we never supported this form of TERM's value.

If you discard the ‘screen.’ prefix by explicitly setting TERM in your
.screenrc; for example:

 term xterm-256color

it makes no difference, i.e. colours are still broken (when COLORTERM
is set to ‘truecolour’).

> Is this something relatively new?

I started using screen in 2011 and I'm pretty sure it was prepending
‘.screen’ then.

I'm not convinced this is the issue.  Remember, the colours were fine
under screen in Emacs 27.2.  There's something in the truecolor
support added in Emacs 28.1, specifically the respect paid to the
vaule of COLORTERM, that doesn't agree with screen.


It seems truecolor support was added to screen in July 2015:

 https://git.savannah.gnu.org/cgit/screen.git/log/?qt=grep&q=truecolor

Debian bullseye ships screen 4.8.0 which, according to:

 https://git.savannah.gnu.org/cgit/screen.git/

was released in 2000 which would suggest my screen does supprt
truecolor.  Oddly, my screen(1) manpage makes no mention of truecolor
support, yet various online manpages, for example:

 https://www.man7.org/linux/man-pages/man1/screen.1.html

document a ‘truecolor on|off’ configuraion option:

 truecolor [on|off]

 Enables truecolor support. Currently autodetection of truecolor
 support cannot be done reliably, as such it's left to user to
 enable. Default is off.  Known terminals that may support it are:
 iTerm2, Konsole, st.  Xterm includes support for truecolor escapes
 but converts them back to indexed 256 color space.

I've added the line:

 truecolor on

to my .screenrc and it doesn't barf, but it makes no diffference
either.  I shall investigate further.