fix(rrweb): queue insertRule events dropped due to mirror race condition

[TueHaulund]

Summary

CSS-in-JS libraries like Emotion (used by Chakra UI v3/v4) create <style> elements and synchronously call CSSStyleSheet.insertRule() before the MutationObserver adds the node to rrweb's mirror. This causes getIdAndStyleId() to return -1 and the style rule events to be silently dropped, resulting in unstyled session replays.

This is a long-standing issue (rrweb-io/rrweb#1230, rrweb-io/rrweb#928) with no upstream fix.

How it works

1. Queue: When insertRule is called on a sheet whose ownerNode is connected to the DOM but not yet in the mirror, queue the {rule, index} instead of silently dropping it.
2. Retry via setTimeout(0): Schedule a single batched retry that fires after the MutationObserver microtask and any remaining synchronous insertRule calls.
3. Deduplicate against _cssText: On retry, check mirror.getMeta(ownerNode) for _cssText. If present, the MutationObserver already serialized the full sheet state — skip the queued events. If absent, emit them as a batched StyleSheetRule callback.
4. Cleanup: Clear the timer and queue on observer teardown.

In the common Emotion case, _cssText always captures all the initial rules, so the retry is effectively a no-op (no duplicate events). The retry acts as a safety net for edge cases where _cssText isn't set (e.g. CORS restrictions on the stylesheet).

Test plan

• Type-check passes (pnpm turbo run check-types --filter=@posthog/rrweb)
• All unit tests pass (89/89)
• Manual test with Emotion/Chakra UI repro app — verify styles render correctly in replay
• Verify no duplicate CSS rules on replay by inspecting recorded events