The dialog that you use to add persistent classes isn't resized correctly.
If you have a lot of classes and try to resize the dialog to view the list better the problem is that the list isn't resized.
If it's possible I'd like to NetFIX  this bug. Is it possible?  http://wiki.netbeans.org/NetFIX
Sergey agreed to review & integrate your patch Joan. Hence I marked this issue accordingly and added it to the list  of NetFIX Candidates. You can start working on it so good luck! :-)
This old bug may not be relevant anymore. If you can still reproduce it in 8.2 development builds please reopen this issue.
Thanks for your cooperation,
NetBeans IDE 8.2 Release Boss