A few months ago I was working on an app where users manage playlists, the kind of thing where people naturally end up with three or four tabs open without thinking about it.
A user added a track to a playlist in one tab, switched to another tab with that same playlist open, and it just wasn't there. Refresh, and it shows up. No refresh, and that tab has no idea anything happened.
It wasn't really a bug. The code did exactly what I told it to. I'd just built the app assuming there was only ever one tab. The moment there were two, neither knew the other existed.
That sent me down a rabbit hole: how do tabs of the same app actually talk to each other? Turns out the playlist thing was just one symptom of a bigger pattern. A cart that doesn't update in another tab. A user logged out in one tab but not another. A background job running five times instead of once.
Here's what I found.
