I'm trying to insert words from the Matchboard via double shift:
assuming that this is the reincarnation of the ??floating match bar.
Instead of opening these mnemonics, the word to the left of the text cursor in the target segment is selected.
Latest Java, latest OS X, keyboard: US Int'l
Yes, that's the feature of the double-shift shortcut. For example, type the first letter (or more letters to filter the matches further) of the match mnemonic and press double-shift on your keyboard.
Aaargh, it's the other way around than I remembered: first the mnemonic THEN the double-shift.
(feeling like a fool now)