
Can anybody say to me, why it can appear? When Thread is working, it happens.

Can anybody say to me, why it can appear? When Thread is working, it happens.
That's because AWT and Swing are rendered in a different manner. It's not recommended mixing both AWT and Swing components.
You can learn more about it here:
http://www.comp.nus.edu.sg/~cs3283/ftp/Java/swingConnect/archive/tech_topics_arch/mixing/mixing.html