Same problem, item (6) of email below, observed with arabtype font.  See
attached.  MWE follows.

\definefontfamily [myfamily] [serif] [arabictypesetting] [features=arabic]
\setupbodyfont    [myfamily]
\setupalign[r2l]
\setupdirections[bidi=global,method=two]

\setupmixedcolumns[register][direction=reverse]
\setupregister[index][
  before=,after=,

  command=\MyCommand,
  alternative=b,
  color=red,
]

\def\MyCommand#1{#1}

%\def\MyCommand#1{\inframed[frame=no]{#1}}  % this fixes the problem

\showboxes

\starttext
\index{آب}
\index{عذر}
\index[تک]{TEX}
testing
\placeindex
\stoptext


any insights about other items in the previous email?

—MHB

On Wed, Aug 17, 2016 at 10:48 PM, Mohammad Hossein Bateni <bateni@gmail.com>
wrote:

> Hello,
>
> For a few days I have been playing with registers to generate an RTL
> index.  Beside sorting, I ran into the following.
>
> 1) How can I get the index section using \subject?  In some styles, I do
> not want to see the section number.
>
> 2) How can I get the Persian comma used instead of the English comma in
> page numbers and also before "see also"?  The first one I can fix by a hack
> but not the second one: defining a custom pagecommand that sets \
> registerpageseparatorsymbol to the Persian comma!  There is a TODO
> comment in strc-reg.mkiv that the comma customization has been planned.
>
> 3) In RTL registers, if an entry starts with an LTR char, the order of
> index entry and page number will be incorrect.  We had similar issues with
> section headings before.  I can "fix" this by setting pagestyle=\zwnj, but
> is there a more principled way to handle this?  See the MWE below.
>
> 4) Is there a way to define the style of "see also" different from
> pagestyle?  What if I want to distinguish "see also" from the actual entry
> this is referring to?
>
> 5) Currently I use \setupmixedcolumns[register][direction=reverse] to get
> the two-column register in the correct order.  What if I have two index
> registers, one for RTL and another for LTR?  Is there a way to set the
> direction key for a specific register?
>
> 6) In a two-column Persian index, using almost any (non-mono) font, some
> letter headings (those extending significantly below the baseline) end up
> with extra space below.  See attached PDF and the MWE.  (I could not
> reproduce this with DejaVuSans or ALM Fixed.)
>
>
> \definefontfamily [myfamily] [serif] [Scheherazade] [features=arabic]
> \setupbodyfont    [myfamily]
> \setupalign[r2l]
> \setupdirections[bidi=global,method=two]
>
> \setupmixedcolumns[register][direction=reverse]
> \setupregister[index][
>   before=,after=,
>
>   command=\MyCommand,
>   alternative=b,
>   color=red,
> ]
>
> \def\MyCommand#1{#1}
>
> %\def\MyCommand#1{\inframed[frame=no]{#1}}  % this fixes the problem
>
> \showboxes
>
> \starttext
> \index{آب}
> \index{عذر}
> \index[تک]{TEX}
> testing
> \placeindex
> \stoptext
>
>
> Some of the setups are to refute suspected causes.  Notice that using
> \inframed fixes the problem.
>
> Thanks,
> Hossein
>