Coming from #2718, might also affect #2661, #2479.

We should investigate how to deal with macOptionIsMeta on MacOS in a unified fashion for keystrokes. Currently its handling is spread over Keyboard.ts, also ppl report issues with accessing 3rd level shift characters. We might need something similar like in iTerm with explicit splitting the left and right option key handling in Meta (left?) and 3rd level access (right?) to overcome those issues.