LaTeX.org

The problem is solved now. The fix comes from personal email from
Richard Koch, who should know! THANKS TO HIM. I give the short fix, then paste his whole email.

Short Fix:
In TeXshop go to the Edit pulldown menu.
Select "Substitutions".
Deselect "Smart quotes".
Done.

The details from the expert ...

Is this a TeXshop issue or a system keyboard issue?
In the Tex preferences I tried changing the font encoding to Unicode - 8 and this
did not change anything I could note.

Another clue.

If I type the symbol under " in my TeXshop editor I see
this has-output symbol (') for one second, and then it changes before my
eyes to this has-no-output symbol (’). But if I cut and paste this
has-output symbol (') it is stable and works.

Thus the only way I can make legible ...

The problem is not gone. And the solutions above don't work!

I have now, starting this week, missing apostrophe's.
To read this comment you really have to squint to see the
problematic symbol.

The basic problem is that when I type right quote (under ")
in TeXshop I get

’ instead of ' . (squint ...