Thanks Eli and Stefan. Your suggestion helped a lot. Missing the ability to see the text that's highlighted can be quite nightmarish. Thankfully it works now.

https://lifeofpenguin.blogspot.com/2022/02/zen-emacs.html

On Wed, Feb 9, 2022 at 9:12 PM Stefan Monnier <monnier@iro.umontreal.ca> wrote:

> Can somebody point me to a piece of code in the display engine which is
> responsible for highlighting text upon selection?

It's not in the display engine (any more).
It's in `redisplay--update-region-highlight`.

Stefan