First reported by Jason Alexander, reproduced by Rick. Example screenshot attached - note that it displays upside down .
The contents of the disambiguation box mostly appear off screen when the user has selected multiple source terms.
1. In the merge authority item tool, source field, search for a keyword that returns multiple terms. (On BotGarden, choose Persons: Rachel. Select seven or more of the terms returned as sources.
2. Enter a term that has multiple matches into the target term field.
3. Roll over the suggested terms to expose the disambiguation box.
This appears to be a general issue with large modals that have AutocompleteInput parameters