Here's an idea — it's high profile (and somewhat obnoxious) but it darn-near guarantees people have the opportunity to see it. A modal that appears and briefly explains what the Timeline does. It could appear regardless of the state of the Timeline — zero state or a populated timeline. We could use a cookie to make sure people don't see it after their initial view. (The hourglass icon is just a placeholder.)

Pros: High visibility, works for all states of the timeline, doesn't waste space in the UI
Cons: Slightly obnoxious, when users close it they won't be able to pull it up again

And another idea: Maybe we roll this into T180089: Interaction Timeline V1: Header & footer and add it to the header.

Pros: Decent visibility, works for all states, unintrusive
Cons: Takes more vertical space, easily skimmed over

@dbarratt @dmaza @kaldari @SPoore @CSindersWMF — thoughts? Other ideas?

+1 for option 2. We could make the text more relevant with css to make sure that catches the eye.

<marquee>Chronologic history for two users on pages where they have both made edits.</marquee>

+1 for option 2 as well, only because I really hate what is effectively an alert and forces the user to acknowledge it before proceeding. The user should always be allowed to ignore it.

Having said that... you could move the alert to the the page (top?) and allow the user to dismiss it if they want.

Let's go for option #2 because it's the simplest at the moment, persistent, doesn't require visual design, and unintrusive. It's already in the acceptance criteria for T180089: Interaction Timeline V1: Header & footer, so I'll close this as a duplicate.