Unclear when user is present vs. in other tab vs. offline

[jkeat]

This PR will show whether the partner's socket is connected or not:
https://github.com/UPchieve/server/pull/371

But a user can have a connected socket and still be unreachable (e.g. they're in another tab). What's the best way to address this?

ideas

• https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API
• read receipts

[thedevelopnik]

The Page Visibility API looks like a good option. It has support back pretty far in pretty much every browser, so not likely to need shims.