On Tahoe (and probably earlier), Accessibility–>Keyboard–>Full Keyboard Access will mess you up. When turned on, it will make certain UI elements (drop-down selections and “…” for sure) to “flash” and be unusable. I spent HOURS on a new Mac Mini last night thinking it was a touchscreen driver problem or something. Then I became convinced it had to be a bug in JUCE and found good evidence that it most certainly is.
This quick video shows you the bad behavior and how to fix.
Many thanks to the very special community member who helped me diagnose and fix this. Every time I interact with the GP community, it has been a wonderful learning experience. I resolve to “give back” to the community more (stay tuned).