Proposal for BiDi in terminal emulators

[Eli Zaretskii via Unicode]

> From: Egmont Koblinger <egmont at gmail.com>
> Date: Thu, 31 Jan 2019 10:21:52 +0100
> Cc: Adam Borowski <kilobyte at angband.pl>, unicode at unicode.org
> 
> > Does anyone know of a terminal emulator which supports isolates?
> 
> GNOME Terminal's (VTE's) current work-in-progress implementation does
> remember BiDi control characters just like it remembers combining
> accents, that is, tied to the preceding letter's cell. It uses FriBidi
> 1.0 for the BiDi work, so yes, it supports Unicode 6.3's isolates.

So we will some day have one such terminal emulator.  That's good, but
a text-mode application that needs to support bidi cannot rely on its
users all having access to that single terminal.

> There's one significant issue, though. Because we currently just
> misuse our existing infrastructure of combining accents for the BiDi
> controls, BiDi controls at the very beginning of a paragraph are
> dropped. Addressing this issue would need core changes to the terminal
> emulation behavior, such as introducing in-between-cells storage, or
> zero-width special characters belonging to a cell _before_ the cell's
> actual letter, or something like this. I outline one idea in my
> specification, but it's subject to discussion to finalize it.

This is indeed a significant issue, because it means applications
cannot force the terminal use a certain non-default base paragraph
direction.