Is your suggestion for improvement related to a problem? Please describe.
When I drag a PDF document into JabRef, it often takes time for it to process it. I also have experienced exceptions for that operation.

Describe the solution you'd like
In general, I like to see some indicator that something is going on in the background (especially when dragging more than one file to the window).

Additional context
I think that Citavi has this (might not be ideal, either, but at least it is there).