Author here :)
I've been using Twin as my everyday terminal emulator and terminal multiplexer since ~2000, slowly adding features as my free time - and other interests - allowed.
As someone pointed out, the look-and-feel reminds Borland Turbo Vision. The reason is simple: I started writing in in the early '90s on DOS with a Borland C compiler, and I used the Borland Turbo Vision look-and-feel as a visual guideline (never actually looked at the code, though).
The porting to linux happened in 1999 (it was basically dormant before that), and Unicode support was progressively added around 2015-2016 (initially UCS-2 i.e. only the lowest 64k codepoints, then full UTF-32 internally, with terminal emulator accepting UTF-8). There are still some missing features, most notably: no grapheme clusters, no fullwidth (asian etc.) support, no right-to-left support.
Right now I'm adding truecolor support (see https://github.com/cosmos72/twin/tree/truecolor) - it's basically finished, I'm ironing out some remaining bugs, and thinking whether wire compatibility with older versions is worth adding.
And yes, documentation has been stalled for a very long time.
Retrospectively, I should have switched C -> C++ much earlier: lots of ugly preprocessor macros accumulated over time, and while I rewrote the C widget hierarchy as C++ classes, several warts remain.
Alas, it's not finished. You've made the mistakes that all of us have made, and haven't caught up with us, must of us having fixed those mistakes, a few years back when implementing 24-bit RGB was in vogue.
This is not, as the function name suggests, a colon, but per ITU/IEC T.416 it should be:
https://github.com/cosmos72/twin/blob/truecolor/server/hw/hw...
And not only should this have colons too, but per ITU/IEC T.416 there's a colour space parameter that goes in there:
https://github.com/cosmos72/twin/blob/truecolor/server/hw/hw...
The unfortunate part is that when rendering to a terminal, you don't have any available mechanism apart from hand-decoding the family part of the TERM environment variable, and knowing who made which mistakes, to determine which of the 7 possible colour mechanisms are supported. They are:
1. ECMA-48 standard 8 colour, SGRs 30 to 37, 39, 40 to 47, and 49
2. AIXTerm 16 colour, ECMA-48 plus SGRs 90 to 97 and 100 to 107
3. XTerm 256 colour, ITU T.416 done wrongly with SGR 38;5;n and SGR 48;5;n
4. XTerm 256 colour corrected, ITU T.416 done right with SGR 38:5:n and SGR 48:5:n
5. 24-bit colour take 1, ITU T.416 done wrongly with SGR 38;2;r;g;b and SGR 48;2;r;g;b
6. 24-bit colour take 2, ITU T.416 done wrongly with SGR 38:2:r:g:b and SGR 48:2:r:g:b
7. 24-bit colour take 3, ITU T.416 done right with SGR 38:2::r:g:b::: and SGR 48:2::r:g:b:::
Few people support 4, and although quite a lot of us have finally got to supporting 7 it isn't quite universal. Egmont Koblinger, I, and others have been spreading the word where we can over the last few years.
This is where I was at in 2019:
https://github.com/jdebp/nosh/blob/trunk/source/TerminalCapa...
There a few updates to that that are going to come out in 1.41, but when it comes to colour they're mainly things like recognizing the "ms-terminal" and "netbsd6" terminal types in the right places.
Do symbols for legacy computing work with it? Especially the 1/8ths vertical/horizontal blocks?