Linux Mint 18.1. This problem seems to be the consequence of annoying design choices, and imposition of "aesthetics", but I'm not sure.
I hate being forced to use the mouse. I consequently use hotkeys and also Alt-[key] a lot. Rather than having to guess what the mnemonic for a menu might be I want to go back to the, er, sensible thing where the menu's mnemonic key is underlined all the time.
At the moment it differs confusingly from app to app.
- For example, on Firefox (v55), which I'm currently using, the underlining is on all the time.
- In Eclipse (Oxygen) there is no underlining ever.
- In Terminal (having enabled mnemonics under Edit-->Prefs) the mnemonics appear when you simply press Alt.
- In xed (default text editor) the underlining appears ever-so-momentarily when you press the full key combination: i.e. once you know the mnemonic it shows you it, not if you just press Alt!
Furthermore I'm not clear whether this behaviour, assuming it can be changed somehow, has to be set on a per-app basis, or whether there's a global setting (or more than one) which governs this behaviour.
There are various solutions out there, e.g. this one (NB Linux Mint is pretty much a fork of Ubuntu), but I haven't found anything which works yet.