Replay: rrweb `Mirror.removeNodeFromMap` never descends into shadow roots, so every removed web component leaks its shadow tree

[mrcrumpy]

Is there an existing issue for this?

• I have checked for existing issues https://github.com/getsentry/sentry-javascript/issues
• I have reviewed the documentation https://docs.sentry.io/
• I am using the latest SDK release https://github.com/getsentry/sentry-javascript/releases
  (observed on 10.63.0; verified that 10.74.0 bundles the identical code via @sentry/rrweb-snapshot 2.43.2, and that 2.44.0 and rrweb master still contain the same method)

How do you use Sentry?

Self-hosted/on-premise

Which SDK are you using?

@sentry/browser

SDK Version

10.63.0 (code unchanged in 10.74.0)

Framework Version

No framework. Server-rendered Symfony app, Stimulus controllers, UI built from Stencil web components (open shadow roots).

Link to Sentry event

n/a (memory growth, no event)

Reproduction Example/SDK Setup

Sentry.init({
  dsn: __YOUR_DSN__,
  integrations: [Sentry.browserTracingIntegration()],
  tracesSampleRate: 1,
  replaysSessionSampleRate: 0,
  replaysOnErrorSampleRate: 1.0,
});

// added later on requestIdleCallback
Sentry.addIntegration(Sentry.replayIntegration({
  maskAllText: true,
  maskAllInputs: true,
  blockAllMedia: true,
}));

Minimal reproduction without any framework:

<script>
  customElements.define('x-box', class extends HTMLElement {
    constructor() {
      super();
      this.attachShadow({ mode: 'open' }).innerHTML =
        '<style>:host{display:block}</style><span><slot></slot></span>';
    }
  });
</script>
<div id="host"></div>
<script>
  // start Replay first (buffer or session mode, both affected), then:
  const host = document.getElementById('host');
  let i = 0;
  setInterval(() => {
    host.innerHTML = '';
    for (let k = 0; k < 100; k++) {
      const el = document.createElement('x-box');
      el.textContent = 'item ' + (i++);
      host.appendChild(el);
    }
  }, 500);
</script>

Steps to Reproduce

1. Start Replay on a page that renders open-shadow-root custom elements (Stencil, Lit, or the plain element above).
2. Re-render a region repeatedly so that shadow hosts are removed and recreated (table paging, list refresh, the interval above).
3. Take a heap snapshot after a few hundred re-renders and look at the retainers of the detached ShadowRoot objects, or count entries of the rrweb Mirror.idNodeMap (the object with idNodeMap: Map and nodeMetaMap: WeakMap).

Expected Result

When a shadow host is removed from the document, all nodes rrweb serialized for it, including the nodes inside its shadow root, are dropped from the mirror. Memory returns to baseline once the removed elements are garbage collected.

Actual Result

Mirror.removeNodeFromMap only recurses over n.childNodes and never looks at n.shadowRoot, so the nodes inside the shadow root of every removed host stay in idNodeMap forever (@sentry/replay/build/npm/esm/index.js, removeNodeFromMap, corresponds to packages/rrweb-snapshot/src/utils.ts in rrweb). Because idNodeMap holds strong references, this pins the detached ShadowRoot, its slots, text nodes, style elements and adopted stylesheets for the lifetime of the recording.

Measured on our production admin app (Chrome 140, Stencil components, one datatable being paged programmatically 50 times per run):

	before	after 50 page changes
live DOM nodes (light + shadow)	~6,000	~6,000
rrweb idNodeMap entries	12,068	13,580
retained ShadowRoot objects	1,640	3,360

Retained JS heap grows about 1 MB per interaction and never comes back; the leaked map entries are exclusively shadow-tree nodes (<slot>, Text, <span>, <style>, nested hosts). Retainer chain from the heap snapshot:

Window.open (patched closure) → context → Mirror (G) → idNodeMap (Map) → <slot> / <style> → ShadowRoot

Users who keep a tab open for a working day end up with tabs of 3 to 4 GB.

A second, smaller consequence: Replay flips from buffer to session mode after the first captured error, so in practice the recorder, and with it this leak, runs for the rest of the session even with replaysSessionSampleRate: 0.

Additional Context

• The gap is in rrweb-snapshot and exists in upstream rrweb-io/rrweb master and in getsentry/rrweb as well; isNativeShadowDom/isShadowRoot helpers are already available in the same file. A fix is to recurse into (n as Element).shadowRoot in removeNodeFromMap in addition to childNodes. Happy to open the matching PR against the fork if you prefer to take it there rather than upstream.
• Related but not covering this: Investigate support for shadow dom + web components #7474 (shadow DOM support investigation), [Replay] Corrupt Replay of website that uses web-components & ionic framework #6615 (corrupt replay with web components), Add DOM node counts to replay SDK #14001 (DOM node counts in replay).

[linear-code]

JS-3745

[andreiborza]

Hi @mrcrumpy, thanks for filing this and adding a reproduction . I created a PR over at our rrweb fork to fix this: getsentry/rrweb#331.

[andreiborza]

This fix will go out with our next rrweb release.