Run the attached code. It creates JFrame with two JComboBoxes. One is on the very bottom so its drop down is opened in a separate window. 0.5f opacity is applied to the window. One JComboBox shows its drop down translucent, the bottom one - opaque. If you maximize the window, the bottom JComboBox will open drop down above itself and in this case it will be transparent. So there is incosistency in the translucency of drop down - it should be either always translucent or always opaque.