The mnemonics underline latch is broken in Modena, but works
properly with Caspian.

On Modena it is 'always-on', but the latching behaviour works as if
it is being toggled. On Caspian it is toggled, and works as expected.

This can be reproduced with HelloMnemonic.
Run with HelloMnemonic with Modena (now the default), and the underline
is always present, but should toggle on Alt presses.
Run HelloMnemonic with -Djavafx.userAgentStylesheetUrl=caspian
and the underline appears and disappears as expected.