FULL PRODUCT VERSION :
jdk1.7.0_04
But the developer preview, too

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.1.7601]
but other Windows too

A DESCRIPTION OF THE PROBLEM :
We call the JOptionPane.showMessageDialog method from an JDialog subclass, it just flickers for a moment and the JDialog continues. The JOptionPane isn't blocking.

It seems to be a problem with the new event queue secondary loop system in 1.7. secondaryLoop.enter() returns immediately. The JDialog starts form the main thread, showMessageDialog from the swing eventqueue.

Also, there exists another JFrame as splashwindow.

REGRESSION. Last worked in version 7u3

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
JOptionPane.showMessageDialog should be shown and block
ACTUAL -
JOptionPane.showMessageDialog flickers and continues

REPRODUCIBILITY :
This bug can be reproduced always.