Proposal for BiDi in terminal emulators

[Egmont Koblinger via Unicode]

Hi,

Terminal emulators are a powerful tool used by many people for various
tasks. Most terminal emulators' bugtracker has a request to add RTL /
BiDi support. Unicode has supported BiDi for about 20 years now.
Still, the intersection of these two fields isn't solved. Even some
Unicode experts have stated over time that no one knows how to do it
properly.

The only documentation I could find (ECMA TR/53) predates the Unicode
BiDi algorithm, and as such no surprise that it doesn't follow the
current state of the art or best practices.

Some terminal emulators decided to run the BiDi algorithm for display
purposes on its lines (rather than paragraphs, uh), not seeing the big
picture that such a behavior turns them into a platform on top of
which it's literally impossible to implement proper BiDi-aware text
editing (vim, emacs, whatever) experience. In turn, vim, emacs and
friends stand there clueless, not knowing how to do BiDi in terminals.

With about 5 years of experience in terminal emulator development, and
some prior BiDi homepage developing experience with the kind mentoring
of one of the BiDi gurus (Aharon, if you're reading this, hi there!),
I decided to tackle this issue. I studied and evaluated the
aforementioned documentation and the behavior of such terminals,
pointed out the problems, and came up with a draft proposal.

My work isn't complete yet. One of the most important pending issues
is to figure out how to track BiDi control characters (e.g. which
character cells they belong to), it is to be addressed in a subsequent
version. But I sincerely hope I managed to get the basics right and
clean enough so that work can begin on implementing proper support in
terminal emulators as well as fullscreen text applications; and as we
gain experience and feedback, extending the spec to address the
missing bits too.

You can find this (draft) specification at [1]. Feedback is welcome –
if it's an actionable one then preferably over there in the project's
bugtracker.

[1] https://terminal-wg.pages.freedesktop.org/bidi/


cheers,
egmont (GNOME Terminal / VTE co-developer)