Note that I’m talking about the GNU Emacs text editor running inside a terminal, and not the terminal emulator contained within Emacs.
I haven’t studied its possible configurations, only the default provided by GNU Emacs-25.2 on Ubuntu 18.10.
It does BiDi on its own, hence with a BiDi-aware terminal emulator RTL strings are reversed back to the incorrect order. The terminal emulator needs to switch to explicit mode. Hopefully Emacs will be adjusted to emit the corresponding escape sequences; until then a wrapper script or a terminfo entry could provide a workaround.
It first runs BiDi on a complete line of a file, shuffles and mirrors the characters accordingly, but then wraps into lines of the terminal emulator as if it was LTR only. This is at least unusual, contrary to UAX #9, and thus the eye sometimes has to move upwards to the previous row of the terminal emulator while reading.
It does not do Arabic shaping.
I haven’t studied their possible configurations, only the default provided by Vim 8.0.1766 and Nvim 0.3.1 on Ubuntu 18.10.
They don’t do BiDi, however, do Arabic shaping. This combination really doesn’t make sense to me.
Even worse, they run the shaping algorithm on the reverse (LTR) order. The logical first letter of a word is shaped to its “final” form, as if it was connected towards its right, that is, as if it was supposed to appear at the left end of a word; and vice versa. No matter if the terminal emulator does BiDi or not, the result is broken.