Please make clear, that filter_horizontal pertains to m2m fields
|Reported by:||jammon||Owned by:||nobody|
|Has patch:||no||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
In the documentation of ModelAdmin options please mention, that filter_horizontal (and filter_vertical) pertains to m2m fields on the edit page.
(I got confused with that option until I realized, that list_filter was what I was looking for.)