How to catch the frozen tabs and jank your monitoring misses

A user's web page froze and all dashboards remained green, despite being a long-lived single-page application that someone keeps open for hours. A customer reported losing work when the page became unresponsive. Monitoring tools that capture sessions are expensive and sample them to control costs, often recording only a fraction of sessions, typically around 10%.

This sampling method only captures sessions that throw JavaScript errors, leaving silent failures unrecorded. Scroll jank, sustained drag stutter, and main thread freezes cannot be detected by these tools, as they only measure user frustration through clicks. To catch these failures, a watchdog must be run in a Web Worker to survive the frozen tab and send a report from a snapshot taken before the tab became unresponsive.

Measuring user-perceived performance with INP (Interaction to Next Paint) misses dead and rage clicks, scroll jank, drags, and the merely-bad interaction. Per-session logs provide a more accurate representation of how users actually complain, as opposed to standard monitoring which aggregates sessions into percentiles and cannot trace back to individual users.