Sorta odd that we have this option, but no documentation on it, and no one (except Apple engineers) know what it does. I had seen that post - it was the only link I found to the exact text of the title.

THANK YOU SO MUCH! I did as you suggested - I rebooted my Mac, and d*mn I can now get it to work!!! I'm running the Thumbnail Extension in Xcode, added breakpoints, and NOW they get hit. BTW - it's just not obvious (IMHO) from the docs, but the thumbnail is NOT the view that is shown in Finder windows to the right of the files (when you select a file). That is a Preview view. The Thumbnails are shown in Finder windows (list or icon view) - so they are pretty small. Maybe there are other views you see them in? PS: using latest Xcode and Mac as of 11/2025