Track posthog-js rrweb divergences to contribute back upstream

[pauldambra]

Track posthog-js rrweb divergences to contribute back upstream

We maintain a divergent fork of rrweb-io/rrweb under packages/rrweb/*. Where a divergence is a general improvement (not a PostHog-product decision), we want to contribute it back so our fork stays thin and upstream benefits.

This replaces the old needs-to-be-contributed-back label workflow from the retired PostHog/posthog-rrweb repo. The label has been recreated here — apply needs-to-be-contributed-back to any future posthog-js PR that adds an rrweb divergence, and add a row below.

Each candidate is assessed against current upstream (2.0.1 + open PRs) as one of: novel (worth opening a PR), in-flight (we already have an open upstream PR — just needs landing), adopted (upstream already fixed it — drop), or product-specific (keep in fork). Companion pull-in tracker: #3765.

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Strongest novel candidates — open these upstream

These are general, low-coupling, and PostHog-agnostic.

Divergence	Source	Upstream status	Notes
CSS var() shorthand <style> textContent preservation	#3542 (rrweb-snapshot/src/utils.ts hasEmptyShorthandLonghand)	novel — fixes OPEN bug	Upstream issue #1667 is open and unfixed; PR #1322 adds only a failing test. Our fix + #1322's test = a ready-made PR. Top priority.
preload-as-style infinite polling/listener leak	#3667 (snapshot.ts stylesheetLoadTracked + resetStylesheetLoadTracking)	novel	Upstream onceStylesheetLoaded has only a local fired flag + timeout — no dedupe Map, no AbortController, no reset. Real leak on SPAs re-snapshotting <link rel=preload as=style>.
Replay scroll re-apply after fast-forward/seek	#3736 (replay/index.ts lastScrollMap, re-applied on Flush)	novel	Upstream replayer does a single scrollTo with no retry; scrolls to not-yet-scrollable targets clamp to 0. Clean standalone replayer fix.
<link> missing-href guard	old #18 (snapshot.ts hrefFrom())	novel	Upstream still does unguarded (n as HTMLLinkElement).href. Tiny defensive fix.
maxDepth cap on DOM serialization	old #130 (snapshot.ts)	novel	Upstream serializeNodeWithId has no depth cap; general crash/payload fix (default 50 + one-time warn). No upstream PR/issue exists.
empty-CDATA rebuild guard	old #70 (rebuild.ts)	novel (overlaps OPEN #1740)	Upstream createCDATASection has no empty guard; OPEN PR #1740 fixes the same area differently (text-node swap). Coordinate / pick one approach.
Angular Zone __symbol__ unpatched-original detection	old #24 (utils/src/index.ts angularZoneUnpatchedAlternative)	novel	Upstream still uses the older isAngularZonePresent(). Could fold into our open #1633.
Replayer destroy() cleanup + idempotency	old #92 (+#122) (replay/index.ts emitterHandlers)	novel	Upstream destroy() only pauses + resets mirrors/media; doesn't track handlers, isn't idempotent, doesn't clear imageMap/canvasEventMap/timeouts.
Plumb dataURLOptions (type/quality) into canvas toDataURL	old #95 (record/observers/canvas/serialize-args.ts)	novel (plumbing only)	Upstream calls toDataURL() with no args. Contribute the plumbing; leave PostHog's webp/q0.4 defaults out.

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Coordinate with in-flight / open PRs (don't open parallel PRs)

Divergence	Source	Related upstream	Action
sheet.href for SPA stylesheet-href stability	#3635 (snapshot.ts transformAttribute/serializeElementNode)	OPEN #1825 (d0ugal, different code path — CSS content, not the href attribute); merged #1705; closed #1700	Frame ours as the companion to #1825 (href attribute vs CSS content). Coordinate with d0ugal.
WebGL context-lost canvas skip	#3527 (canvas-manager.ts isContextLost() pre-flight + getCanvas try/catch)	OPEN #1469 (daibhin, catches createImageBitmap errors); merged #1777 (unrelated WebGL-exists guard)	Ours is the better mechanism (skip lost contexts before they poison the fingerprint-dedup map). Land jointly with / on top of #1469.
iframe mutation-buffer cleanup race	#3558 (observer.ts findAndRemoveIframeBuffer(knownDocs) + attachedDocuments)	OPEN #1755 (megboehlert)	Tightly coupled with #3570 below.
same-origin iframe state leak on removal	#3570 (record/index.ts iframeObserverCleanups; IframeManager.destroy())	seeds OPEN #1755; prior art closed #1746/#1747	Contribute #3558+#3570 as one combined iframe-lifecycle PR that extends #1755.
iframe attach/re-attach + lifecycle cleanup cluster	old #108, #121, #123, #142, #91, #94, #100 (iframe-manager.ts, record/index.ts)	addresses upstream issue #1720; overlaps external OPEN #1791 "Address Iframe memory leaks"	Same surface as #1755/#1791. Bundle into the combined iframe-lifecycle PR rather than many small ones.

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Our PRs already open / merged / closed upstream

PostHog-authored (or PostHog-seeded) PRs on rrweb-io/rrweb. Verified individually by PR number (the multi-author search index is unreliable cross-repo).

PR	State	Corresponds to	Next action
#1597	merged	Angular wrapped MutationObserver detection (pre-fork)	Done — in our fork base.
#1705	merged	node.baseURI for stylesheet hrefs (related to #3635)	Done. Our #3635 layers on top.
#1633	open	untainted prototype access in Angular	Nudge to land.
#1635	open	iframe + custom web components (Chrome)	Nudge to land.
#1641	open	preserve adopted styles (virtual dom)	Nudge to land.
#1686	open	stringifyRule error swallowing	Reflected in our utils.ts (cites #1686). Nudge to land.
#1712	open (APPROVED)	console.log -> suppressible warn	Nudge for merge.
#1755	open	iframe pagehide buffer/mirror cleanup -> seeds #3558+#3570	Extend into the combined iframe-lifecycle PR.
#1806	open	virtual-dom backward-skip playback	Nudge to land (also a pull-in item — not yet vendored).
#1814	open	untainted add/remove listeners	Nudge to land.
#1825	open	link.sheet for CSS capture on WebKit (companion to #3635)	Coordinate with #3635.
#1826	open	Zone.js unpatched MutationObserver on Safari	Nudge to land.
#1685	closed	stringifyRule (superseded by #1686)	None.
#1700	closed	document.baseURI sheetHref (superseded by #1705)	None.
#1746 / #1747	closed	iframe mem-leak (prior art for #3570)	Cite as prior art in the combined PR.
#1818	closed	swallow CSS insertRule parse errors	Decide whether to re-propose.

Possibly-ours (high-trust contributors — confirm affiliation before claiming): #1642 (kevinatown, blockElementFn), #1469 (daibhin, bitmap errors), #1769 / #1771 (heathdutton, seek custom events / replay autocomplete).

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Adopted upstream / product-specific — drop or carve out

Item	Source	Verdict
applyStyleDeclaration null-safety	old #86	Adopted upstream via merged #1775. Drop.
splitCssText/normalizeCssString caching	old #17	Gone from our fork (lost in an upstream re-sync; upstream replaced it with 0px-normalization). Moot.
Don't bundle css parsing / postcss into recorder	old #96, #101	Packaging-specific (@posthog/rrweb-snapshot/record subpath exports + our vite config). Keep fork-only. (Upstream's own version is OPEN #1784.)
base64 image recompression + striped-placeholder	new (maxBase64ImageLength, recompressBase64Image, STRIPED_PLACEHOLDER_SVG) + old #99	Product/storage-cost policy (lossy webp q0.4 + fixed placeholder). Keep in fork; at most offer upstream as opt-in config.
ph-no-capture blocked-element layout fix	#3678	Carve out: contribute the general rr_position/rr_transform/rr_display flow-preservation; drop the ph_rr_could_not_detect_modal diagnostic attribute (PostHog-specific).
CanvasManager reference-counting teardown	#3726	Novel and correct, but higher coupling — changes the manager lifecycle contract (every per-root teardown must acquire()/reset() in pairs). Get a maintainer design nod before opening a PR; don't lead with it.

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Summary

• 20 historical needs-to-be-contributed-back PRs + 10 post-move divergences assessed against current upstream.
• ~9 novel candidates worth opening PRs (top: fix(replay): preserve <style> textContent on var-shorthand CSS #3542 CSS var(), fix(replay): stop polling preload-as-style <link> elements forever #3667 preload leak, fix(replay): re-apply scroll after fast-forward catch-up #3736 scroll re-apply).
• ~12 of our PRs already open upstream — the highest-leverage move is nudging these to land (refactor: Simplify Posthog.init() signature #1712 is already APPROVED).
• iframe-lifecycle work (new fix: guard iframe buffer cleanup race #3558/fix(record): stop leaking same-origin iframe state on removal #3570 + old Improve session recording #91/Refactoring: extract code from autocapture.js, add tests #94/Polling and feature_flag_called event spam #100/Allow toggling of feature_flag_called #108/Don't truncate strings for session recording #121/Handle cycles in objects in posthog.capture #123/Capture information on failed/in-flight requests #142) should land as one combined PR coordinated with fix: check url matching on URL change for widget-type surveys #1755 / feat: Block yet another semrush crawler/bot #1791, not a dozen small ones.
• Drop: Add $pageview bounce threshold setting #86 (adopted), Cookie warning on Chrome #17 (gone), packaging (Send/create $session_id on $snapshot events #96/$snapshot events being sent for users that opted out #101), image recompression (product); carve ph_* out of fix(replay): fix absolute position bug in ph-no-capture #3678.

Assessed against upstream rrweb-io/rrweb on 2026-06-08; re-verify a row's status before acting on it.