Source Database of a restore stuck Restoring
This can happen when you don't uncheck in the GUI to take a tail-log backup.
It is a nasty behavior by the GUI. You type a new database name in the restore GUI, the one to be created by the RESTORE command. But the GUI still does a tail-log backup of the source database, and it does it by default using the NORECOVERY option (which s precicely what puts the source db in restoring state).
The image you posted doesn't reflect this, so I can deduce that it wasn't the actual screen from your restore. You would have a note in the upper left of the dialog.
So, in the future, go to the Options page and uncheck the tail log backup when you restore into a new database name.
To handle the immediate situation, you just execute below (obviously substituting the database name):
RESTORE DATABASE dbname WITH RECOVERY

No backup files needed.