You should create and show Swing windows/components only in the Event Dispatch Thread. Check this link which shows a sample program from the Oracle tutorial: sample.
You have to wrap the calls in a SwingUtilities.invokeLater().
SCJP 1.4, OCMJEA/SCEA 5.0.
Joined: Sep 19, 2012
Ok, so I understand, after reading about Initial Threads that I should create GUI elements like that, letting that work be done at the end by a thread which is going to be invoked when possible.
So, that symbol is appearing because my windows 7 tablet is single core and can't create gui then? It sounds right, but at my code I'm doing it at the beginning, so I don't understand how this problem appears in this particular case.
The problem you are having usually happens when the window is resized so the component can't fit it so it "shrinks" (though your frame is set not to be resizeable, which confuses me now).
Try adding this: