We have just upgraded a site from EPiServer version 7.1 to 9.x.
In the old version we had a custom category selector (where we hided some of the "category groups" depending on what page type we were editing).
We were using a custom EditorDescriptor for CategoryList that were pointing out custom js-files in the ClientResources folder.
The custom js-files were rewritten js-files that we found in the EPiServer installation folder (C:\Program Files (x86)\EPiServer\CMS\7.0.586.1\Application\UI\EPiServer\CMS\ClientResources\EPi\Cms\widget).
How is it supposed to be solved in EPiServer 9. The same way? Where do I find the js-files then?
Probably the same way. Start looking in
for your js file. That's my best guess.