Mnemonic underlines displayed in Swing applications under the GTK L&F are lower than
in native GTK applications. The attached screenshots demonstrate the buggy Java
behavior (menu-java.png) and the correct native behavior (menu-native.png). These
screenshots were taken on Nevada b27a with the Blueprint theme, but similar behavior
can be seen on S10 and various Linux systems.
I've attached another screenshot (checkbox-rollover-native.png) that shows how
native GTK renders mnemonic underlines for characters that have a descender
(in this case a lowercase 'y').