Recorder no longer freezes UI on CSS-heavy pages

1.412.0

Minor Changes

• #4320 837363e Thanks @posthog! - Bound the main-thread cost of the recorder's initial full snapshot, and start measuring it.

  stringifyStylesheet reads cssText for every CSSRule of every stylesheet, all inside the one uninterruptible task that takes the full snapshot. On CSS-heavy pages that is the dominant cost, and it has been observed freezing the UI (no rendering, scrolling, or cursor movement) for seconds. The existing maxDepth guard doesn't help, since it only bounds deep DOMs, not wide ones or heavy CSS.

  Stylesheet inlining now stops after a budget of CSSRules per snapshot (session_recording.inlineStylesheetBudgetRules, default 10,000; set 0 for the previous unbounded behaviour). Sheets past the budget are serialized without _cssText, keeping rel/href so replay can load them remotely, and are then inlined one per idle callback and delivered as attribute mutations. Replay fidelity is preserved; the work is just no longer one long blocking task.

  The snapshot's cost is also now measured and reported on captured events, so slow snapshots are visible without a browser profile: $sdk_debug_replay_slowest_full_snapshot_ms, $sdk_debug_replay_slowest_full_snapshot_stylesheet_ms, $sdk_debug_replay_slowest_full_snapshot_nodes, $sdk_debug_replay_slowest_full_snapshot_css_rules, $sdk_debug_replay_deferred_stylesheets, and, for the incremental path, $sdk_debug_replay_slowest_mutation_batch_ms. (2026-08-05)