✨ Add tab.id to all RUM and Logs events

[mormubis]

Motivation

The Browser SDK shares session state across tabs via cookies/localStorage, but has no way to identify which specific tab produced a given event. Adding tab.id enables distinguishing events from different tabs within the same session, enabling tab-level correlation between RUM and Logs.

Changes

• New tabContext module (packages/core/src/domain/contexts/tabContext.ts): Generates a UUID v4 per browser tab, persisted in sessionStorage so it survives same-tab navigations and reloads. Falls back to an in-memory UUID (cached at module level) when sessionStorage is unavailable (Web Workers, sandboxed iframes, quota exceeded), ensuring RUM and Logs always share the same tab.id within a page lifecycle.
• Wired into RUM and Logs: Both SDKs call startTabContext(hooks) which registers an Assemble hook contributing { tab: { id: "<uuid>" } } to every event.
• Unit tests: Hook output, ID consistency across triggers, sessionStorage persistence/reuse, fallback sharing across SDK instances, and error paths (getItem throws, setItem throws).

Field placement

tab.id sits at the top level alongside view, session, usr, etc. — it has a different lifecycle than a view (a single tab spans multiple views via SPA navigation) and is customer-facing, unlike the _dd internal namespace.

Note: A schema PR in rum-events-format is required to formally define tab in the event schema. That is tracked separately; the field is emitted correctly today via the open-ended index signatures in the generated type files.

Behavior

Scenario	Behavior
New tab	Generates UUID, persists to sessionStorage
Page reload / same-tab navigation	Reuses UUID from sessionStorage
sessionStorage unavailable	Shared in-memory UUID for the page lifecycle
Tab duplication	Both tabs share the same ID (known tradeoff — inherent to sessionStorage-based schemes)

Test instructions

yarn test:unit --spec packages/core/src/domain/contexts/tabContext.spec.ts
yarn test:unit --spec packages/rum-core/src/domain/assembly.spec.ts
yarn test:unit --spec packages/logs/src/boot/startLogs.spec.ts
yarn typecheck

Manual verification (sandbox):

1. Run yarn dev → open http://localhost:8080 in two separate tabs
2. In each tab's DevTools console, paste:

   window.__ddBrowserSdkExtensionCallback = (msg) => {
     if (msg.type === 'rum' || msg.type === 'logs')
       console.log(msg.type, msg.payload.type ?? 'log', '→ tab.id:', msg.payload.tab?.id)
   }

3. Observe: each tab emits a different UUID; reloading a tab preserves its UUID.

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	173.86 KiB	174.12 KiB	+273 B	+0.15%	✅
Rum Profiler	4.71 KiB	4.71 KiB	0 B	0.00%	✅
Rum Recorder	24.88 KiB	24.88 KiB	0 B	0.00%	✅
Logs	56.32 KiB	56.58 KiB	+271 B	+0.47%	✅
Flagging	944 B	944 B	0 B	0.00%	✅
Rum Slim	129.58 KiB	129.85 KiB	+271 B	+0.20%	✅
Worker	23.63 KiB	23.63 KiB	0 B	0.00%	✅

🚀 CPU Performance

Action Name	Base CPU Time (ms)	Local CPU Time (ms)	𝚫%
RUM - add global context	0.0055	0.0038	-30.91%
RUM - add action	0.022	0.0121	-45.00%
RUM - add error	0.0202	0.0117	-42.08%
RUM - add timing	0.0038	0.0025	-34.21%
RUM - start view	0.0157	0.0113	-28.03%
RUM - start/stop session replay recording	0.0008	0.0007	-12.50%
Logs - log message	0.0282	0.0141	-50.00%

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	26.69 KiB	27.72 KiB	+1.03 KiB
RUM - add action	49.54 KiB	51.45 KiB	+1.91 KiB
RUM - add timing	26.97 KiB	25.80 KiB	-1.17 KiB
RUM - add error	53.05 KiB	56.57 KiB	+3.52 KiB
RUM - start/stop session replay recording	25.24 KiB	25.03 KiB	-208 B
RUM - start view	448.38 KiB	459.28 KiB	+10.90 KiB
Logs - log message	44.46 KiB	45.06 KiB	+618 B

🔗 RealWorld

[datadog-datadog-prod-us1]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 82.35%
• Overall Coverage: 77.15% (-0.00%)

_{This comment will be updated automatically if new data arrives.
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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reuse fallback tab ID across SDK initializations

When sessionStorage is unavailable, retrieveOrCreateTabId() returns a fresh UUID on every call. Since both startRumEventCollection() and startLogs() call startTabContext(), pages that initialize both SDKs in environments where storage throws (for example sandboxed iframes or restricted privacy modes) will emit different _dd.browser_tab_id values for RUM vs Logs from the same tab, breaking tab-level correlation. The fallback path should cache one in-memory ID per page lifecycle instead of regenerating it per initialization.

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[chatgpt-codex-connector]

Keep INP sub_parts in exported RUM event typings

This deletion removes view.performance.inp.sub_parts from the public RumEvent type, but INP sub-parts are still produced at runtime in viewCollection (sub_parts is assigned when INP subparts exist). That creates a TypeScript API regression: consumers that previously read inp.sub_parts now get type errors even though the SDK still sends the field. The generated type should remain aligned with emitted payloads (including the correct processing_duration key).

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[BenoitZugmeyer]

The new field should be added in rum-events-format