Add "fallback" functionality for the custom mapping
Add "fallback" functionality for the custom mapping as a feature (#262707 (comment 484515973))
It looks like a small separate feature, and I think we can slightly reduce the scope and release the first version of the custom mapping without the fallback support.
There is no need to redesign that part of the interface, it seems to me the fallback column can be easily hidden from the UI without any harm to UX.