>
> It is a side effect of what the the par builder considers to be valid
> breakpoints. The current approach is playing very safe but after looking at
> it Taco and I decided that it can be a more tolerant with respect to end
> dirs so the next luatex version will have that.
>
> Anyway: You need to code carefully: the space after "TRT" in "\textdir TRT
> x" is meaningful so in your example you introduce spaces.
>

Thanks for the explanation.  So the space after \textdir TLT is meaningful
but the one after \lefttoright isn't.  (I guess, I should have used
\textdir TLT\relax.  I'm going to use the high-level \lefttoright command
from now on.)

Also, in context don't use \textdir etc directly, just use \lefttoright and
> \righttoleft in combination with \setupalign as I will not spend much time
> on side effects of interfering with these low level dir changers directly.
>

Great!  You partly answered another question I meant to ask: what is the
proper way to write \textdir and \pardir in CONTEXT?  So \lefttoright and
\righttoleft are replacements for \textdir TLT and \textdir TRT.  How
should I code in \pardir TRT in CONTEXT?  I couldn't find anything for that
in spac-ali.mkiv.

Those low-level LuaTeX directives were part of an attempt to come to the
core of a problem I'd run into with numbers at the end of a right-to-left
line.  The BiDi algorithm correctly gave left-to-right direction to that
digit sequence and the result was that the word following the number had
gone into the margin.

—MHB