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The background of a JComponent gets painted in its paint method.
Your PaintSurface class overrides that method, without containing a call to super.paint(Graphics) or drawing the background itself. So the background doesn't get painted.
Does that help?
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus