Thank you all for testing your installation.
Getting mysterious.
Using Rstudio build 492:
With Project 1, hover doesn't work.
With Project 2, hover does work.... if opened in a fresh session
... but not if opened through "Open Recent Projects".
Then, close and re-open, Project 2 loads, and hover works.
Then open Project 1 through "Open Recent Projects", doesn't work.
Finally, close RStudio and re-open on Project 1,
... because maybe it's the fault of "Open Recent Projects"...
but no, hover not working.
And finally finally, open Project 1 in a fresh pre-492 version (443),
hover not working,
but opening Project 2 via "Open Recent Projects", it DOES work.
So all of my hypotheses are bust.
Curse the day that we lost our beautiful Mathtype,
Apple went full 64-bit-only and Wiris didn't come through
with a usable replacement.
Casting about ever since.
None as good convenient as hover preview in Rmarkdown!
(Aside: Spellcheck hover works if "Use real-time spellchecking" is on.
So it's not the hover mechanism.)