How Frozen Tabs and Silent Jank Slip Past Standard Web Monitoring Tools

Modern real user monitoring (RUM) and session replay tools often miss critical performance failures because they sample only a fraction of sessions and rely on JavaScript errors to trigger full capture. Silent failures such as frozen tabs, scroll jank, and drag stutter throw no errors, so they are rarely recorded unless they happen to fall within the sampled portion. Long-lived single-page app sessions compound the problem, as replay length is typically capped, meaning the slow buildup before a freeze may never be stored. A Web Worker-based watchdog can detect main-thread freezes independently, since a frozen main thread cannot report its own failure. Per-session logging built on self-owned infrastructure is proposed as a way to ensure every complained-about session remains traceable.
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