Windows L&F doesn't show mnemonics in menus by default, to see them you need to press Alt,
but this doesn't work for popup menus since Alt closes them. And this is usability
issue :(
I've tried native apps and found that WindowsXP shows no mnemonics in popups if they are shown
in response to mouse, but it shows mnemonics if popup is shown by "context menu" key.
The same functionality should be implemented in Swing, if user reister popup menu using
JComponent.setComponentPopupMenu().