The JComboBox popup window appears 3 pixels wider than the combo box. BasicComboPopup.getPopupLocation attempts to set the width of the popup to the width of the combo box. Instrumenting that method shows that both of these widths are reported to be the same. However, once drawn, the popup window appears wider.

The problem is evident in for all look and feels.

While this is a small visual problem for left-to-right locales, it is a much more glaring problem for right-to-left locales. The visual effect is that the combo box and the popup are aligned on their left edges rather than their right.