I'd like to see each hotkey character on my "Menu" toolbar and its submenus underlined all the time. Presently, I only see these underlined when I toggle the hotkey display with a press of the Alt key.
I searched Opus's settings, Google and the 12.3.3 help file, but I didn't find this.
That's actually a Windows setting, not an Opus one.
See Make Windows Underline Menu Shortcut Keys for instructions on changing it.
You're correct, of course. I tested this with one Microsoft application before I created this discussion threat, to ensure I had the Windows 10 setting to always underline the hotkeys turned ON. In that Microsoft application, the underlines were always showing. Pressing Alt had no impact on them. So I decided I had the Windows option ON.
It was tricky for me to find a Microsoft application with a menu bar instead of a ribbon. Nevertheless, I clearly chose this test application poorly. It was Word 97. I guess its hotkeys are always displayed/underlined. 
"thread" not "threat".
Wowzers. Word 97.
We keep it around for some business software that ties into it. We also use Word 2016. I picked it because I knew it'd have a menu bar.
Notepad is probably a good one to test with. It still uses menus, although I am sure it is only a matter of time. 
Good call.