This is still/again broken. Now arrow keys are ignored if Num-Lock or Caps-Lock is active.
Ah so it's due to NumLock being active, I see. I couldn't understand why someone else said Down Arrow worked for selecting engine. At least now I know :face_with_stuck-out_tongue_closed_eyes:
With NumLock on you can actually press Shift+Down Arrow to get the history and Tab will select. But it should be Arrow keys. And still can't delete entries with keyboard.
Also I believe the best would be to have Down Arrow to select history, and Shift+Down Arrow to select engine. Because using search history is usually the most common thing done, while selecting active engine is rarer.
If placed on the shortcut works only from there; if placed as flags is bind to the Vivaldi installation. It's matter of preference.
--flag-switches-begin [...] --flag-switches-end could be omitted: is a place-holder to recognize which command lines are add from users and which from vivaldi itself.
I don't see --silent-debugger-extension-api [Link] from flags. But usually debug-packed-apps [link] is sufficient.