This is observed on Nimbus L&F on all OSes, with the current 6u4b02 pit build.
There are multiple issues in Menus/PopupMenus in Nimbus L&F.

1)There is too much space (top and bottom insets) around the submenus and menu items. This is clearly visible for - Above the first menu or Below the last menu in a drop down menu. Screen shot are attached for refenrece.
Actual: See NimbusL&F_PopupMenu.png - There is too much space above and below the menu item with text "Popup Submenu -1" AND There is too much space below the menu item with text "Popup Submenu -11".
Expected: See GTKL&F_PopupMenu.png,GTKL&F_Popup_Selection.png - The correct behavior in GTK L&F with nimbus theme. Observe that there is very little space around the menu text "Popup SubMenu-1".

2)Accelerator of Disabled MenuItems is enabled. It should be disabled.
Actual: See NimbusL&F_Menu_Accelerators.PNG, NimbusL&F_PopupMenu.png - The Accelerator "Alt+4" for "PopupMenuItem -4" is enabled, even though the menu item itself is disabled.
Expected: See GTKL&F_PopupMenu.png -The correct behavior in GTK L&F with Nimbus theme. Accelerator "Alt+4" for "PopupMenuItem -4" is disabled.

3)Selection appears as a band touching edges of the menu. It should be a rectangular shape within the bounds of the menu.
Actual: See NimbusL&F_Popup_Selection.png- The selection color starts and ends at the menu edges.
Expected: See GTKL&F_Popup_Selection.png - The correct behavior in GTK L&F with Nimbus theme. Selection does not touch the edges of the menu border.

4)This is similar to previous issue(3) Separators in the menu touch the edges of the menu while it should actuially be smaller in length than the menu.
Actual: See Nimbus_Seperator Issue.PNG Separator edges extend to the end of the menu border.
Expected: See GTK_Seperator.PNG - The correct behavior in GTK L&F with Nimbus theme. Separator does not touch the edges of the menu border.