Product Version       = NetBeans IDE Dev (Build
200406151800)
  Operating System      = Linux version
2.4.22-1.2115.nptl running on i386
  Java; VM; Vendor      = 1.5.0-beta3; Java
HotSpot(TM) Client VM 1.5.0-beta3-b55; Sun
Microsystems Inc.
  Java Home             = /usr/local/java/jdk1.5.0/jre
  System Locale; Encod. = cs_CZ (nb); ISO-8859-2
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Customization of J2SE App project type, especialy
setting Main Class is done in AWT Thread which has
side effect that user might think that IDE freez.

See Thread Dump

Reporduction:
1)Have J2SEE App project. Don't generate Main
class when you creating it.
2)Have some sources with some class containing
Main method
3) Invoke Properties on your Project node.
4) Under the node Run, select node Running Project
   and click to BRowse button of Main Class.

5) Your sources are scanned in blocking AWT (not
in background) to help you identify Main Class of
the project. 

You have to wait very long time till a dialog
where you could select main class is brought up.