Basically, Swing only understand how to paint transparent and opaque components, it doesn't know how to deal with translucent components, so using an alpha based background color causes issues.
Instead, you need to "fake" it by taking control over how the component's background is painted, for example...
public class FakeTransperencyButton extends JButton {
private float alpha = 0;
public FakeTransperencyButton(String text) {
super(text);
setOpaque(false);
setBackground(Color.RED);
addMouseListener(new MouseAdapter() {
@Override
public void mouseEntered(MouseEvent e) {
alpha = 0.4f;
repaint();
}
@Override
public void mouseExited(MouseEvent e) {
alpha = 0f;
repaint();
}
});
}
@Override
public boolean isOpaque() {
return false;
}
public float getAlpha() {
return alpha;
}
protected void paintComponent(Graphics g) {
Graphics2D g2d = (Graphics2D) g.create();
g2d.setComposite(AlphaComposite.SrcOver.derive(getAlpha()));
g2d.setColor(getBackground());
g2d.fillRect(0, 0, getWidth(), getHeight());
g2d.dispose();
super.paintComponent(g);
}
}
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