An automation surface that is hidden delivers no animation frames, so a rAF-driven app cannot be driven through it

Fact

Browser automation panes, headless surfaces and background tabs report document.hidden === true, and a hidden document is throttled until requestAnimationFrame stops firing altogether. Any app whose state advances inside its rAF loop is therefore frozen for the whole session: input is accepted, the state machine never runs, and clocks stop.

Screenshots still return an image, because a capture forces a paint of whatever was last committed. So the surface looks alive while nothing is simulating, and the state you read back is whatever it was when the pane went hidden.

The tell is an internal clock that will not move: sample the app's own accumulated time twice across a real delay, and it reads identical.

An injected script that waits for a frame does not return a wrong answer — it never returns at all. await new Promise(r => requestAnimationFrame(r)) inside an evaluate call hangs until the automation tool's own timeout fires, which also abandons every remaining step of a batched call.

Why it matters

Every conclusion drawn this way is wrong in the same direction — the feature looks broken because the loop never ran. Time then goes into debugging input handling, event wiring or state transitions that are all correct, and the "fix" is validated against the next equally frozen run.

innerWidth/innerHeight can also read 0 on a hidden pane, which quietly satisfies (orientation: portrait) and any max-width query, so layout probes taken there report a device shape that does not exist.

How to apply

• Check document.hidden before concluding anything about a running loop.
• Never await a frame in an injected script. Settle on a timer, or simply take the next action; a timer still fires where a frame does not.
• Expose a deterministic step for automation — a function that runs N fixed timesteps of update and then draws once. Driving that is faster and more reproducible than waiting on real frames, and it makes the whole run seedable.
• Remove the hook before shipping, or gate it behind an explicit flag.
• Read state from the app, not from the picture; see [[screenshot-lags-committed-dom-state]]. The same throttling seen from inside the app is [[animation-lifetime-in-frames]].