Phone Focus Mode is Sidecar's way of keeping you honest about actually looking at your phone while you're supposed to be focused — leave the tab and it tells on you, loudly, on any big screen showing your Context View.
Sidecar → Focus tab → the 📵 Phone Focus Mode switch. Off, it just nudges you if you navigate away; on, it warns: "put your phone down — wander off and your big screen goes red."
Leaving or backgrounding the Sidecar browser tab — detected the moment it loses visibility — while Phone Focus Mode is on and a focus is active.
A local notification: "👀 Back to it" — "You stepped away from Tabatha while in Focus Mode. Eyes back on the task 👇"
If a paired Context View is showing, it turns into a red "📵 Put the phone down" screen: "You stepped away from focus — back to it."
No Context View paired yet? Start here: Set up Context View.Extension Settings → 📺 Context View → "Phone-away fade" → Slow fade (7 seconds) or Immediate. Sidecar's own Settings screen has an equivalent "Immediate phone-away alert" switch, in case the phone is what's handy.