Minor point: changing "\pardir TLT" to "\pardir TLT\textdir TLT" in the
last paragraph produces better visual, however, the previous paragraphs
already demonstrate the problem sufficiently.

On Fri, Jan 8, 2016 at 10:40 PM, Mohammad Hossein Bateni <bateni@gmail.com>
wrote:

> Hello,
>
> When a line ends with a sequence whose direction differs from that of the
> paragraph, we risk pushing some text into the margin (when not necessary).
> Here is an example with corresponding output:
>
>
> \usemodule[simplefonts]
> \setmainfont[ALM Fixed][features=arabic,range=arabic]
> \setupalign[r2l]
> \setupwhitespace[big]
> \showframe
>
> \starttext
>
> % 10 copies of Persian word "hello" stay on one line.
> \dorecurse{10}{سلام }
> % 20 copies makes a 2-line paragraph.
> \dorecurse{20}{سلام }
>
> % one copy of the word goes into the margin although the Latin letters
> perfectly fit the line.
> \dorecurse{10}{سلام }
> {\textdir TLT\dorecurse{20}{a}}
> \dorecurse{10}{سلام }
>
> % although the Latin string extends into the margin, TeX still puts one
> copy of "hello" there as well.
> \dorecurse{10}{سلام }
> {\textdir TLT\dorecurse{30}{a}}
> \dorecurse{10}{سلام }
>
> % something similar happens here with the opposite par/text dir
> \pardir TLT
> \dorecurse{10}{bidi }
> {\textdir TRT\dorecurse{20}{آ}}
> \dorecurse{10}{bidi }
>
> \stoptext
>
>
> The problem seems to be that after typesetting the LTR text within the RTL
> paragraph, TeX thinks the current text ends at the left end of the LTR
> portion; hence, it tries to add something to the line; it's only after that
> that it discovers we ran into the margin!
>
> —MHB
>
>