This may be an off the cuff idea but I've checked on if this seems to work using Geta Categories https://github.com/Geta/EpiCategories which is a replacement package for the in-built category system allowing categories to be managed in the edit area instead of admin with a standard content editing API. I guess it depends if you use categories heavily already or this is a new use.

In this package the category field is a separate fileld and can be marked as [Required] so should support your requirements.

Thanks Scott, Categories is heavily used on this site already.  But that is an inteersting thought for future projects.

Still seems like an Epi bug not to be able to see the categories on that pop-over at the very least.

By the pop over, are you referring to the create window or are you using the quick edit feature in either BlockEnhancements labs package or the later CMS release.

The the create window on a recent version of CMS (we recently updated to 11.20.3), the one showing in the screenshot above, if it shows for you.

Yes, just checking. That's something that's been in the blockenhancement package for a while and just been added to CMS core

To close out this topic, I ended up creating a property to determine how may thumbnails to show.  It was not required and could have been hard coded as we always want the same number.  Ironically I setup a default for the property for the value we wanted.  But that did not work either because the enabling of the Save button required a change in a control.  So if the pop-over screen had the right value, you still could not save and close the window.

Its definitly a use case that Episerver did not consider and should be a quick bug fix.