If you do this:
1) Make sure your User Library folder is empty
2) Open an FXML with a custom component
    => a unresolved component is reported : OK
3) Drag the corresponding JAR on the Library panel and click Import
    => Library panel copies the JAR in the User Library folder : OK
    => the component remains unresolved : BUG

However, in step #2, if you drop the JAR directly in the User Library folder (i.e. using Finder/Explorer),
then the component is correctly resolved.

So it seems that we have a regression in the Library Import dialog.