Collaborative document focus indicator

[smokris]

During a collaborative session, it would be helpful to have a way to indicate to all participants which section of the document that we're currently focusing on / discussing / collaboratively editing.

For example, if we're taking notes during a meeting, and someone has to step away for a few minutes then rejoins the meeting, they could look for the focus indicator to quickly find where we are now.

The existing behavior — showing cursor positions — is not ideal, since people may casually click in other areas of the document, or may just leave their cursor in the position they last typed text, which doesn't necessarily indicate where the discussion currently is happening. Having a loud, intentional position indicator, which anyone (or only someone with a "moderator"-type role) can modify (drag upward/downward), could help better convey discussion status.

A couple quick mockups:

existing behavior	mockup: dim other areas	mockup: big arrow