Otared, Luigi—many thanks for your help. I read the changes file—I neglected to do this when I downloaded TeXShop 3.89— and have added the line 
% !TEX useOldSyncParser
into my source file.  Synctex is now working as it should (with the latest ConTeXt and TeXShop).

All best, Alan

On Fri, Nov 17, 2017 at 8:31 AM, luigi scarso <luigi.scarso@gmail.com> wrote:
On Fri, Nov 17, 2017 at 2:27 PM, Otared Kavian <otared@gmail.com> wrote:
> Hi Alan,
>
> A few weeks ago Steffen Wolfrum and myself reported the issue to Richard
> Koch who did some research and discovered the origin of the problem. He has
> written again the part of the code related to synctex and ConTeXt and will
> issue a new version where the problem is fixed.
>
> In the meanwhile you can use a previous version of TeXShop (I use version 77
> and I can send you a Dropbox link to download it, of you wish) where the
> syncing between the PDF result and the ConTeXt source works fine.
>
> Best regards: OK
>


http://pages.uoregon.edu/koch/texshop/changes_3.html


--
luigi
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