✨ Add browser 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 _dd.browser_tab_id enables distinguishing events from different tabs within the same session.

Changes

• New tabContext module (packages/core/src/domain/contexts/tabContext.ts): Generates a UUID v4 per browser tab, persisted in sessionStorage so it survives page reloads. Falls back to in-memory UUID when sessionStorage is unavailable (Web Workers, sandboxed iframes).
• Wired into RUM and Logs: Both SDKs register the tab context via the Assemble hook, so every event includes _dd.browser_tab_id.
• Unit tests: Covering hook output, ID consistency, sessionStorage persistence/reuse, and error fallbacks (both getItem and setItem failures).

Why _dd?

browser_tab_id lives under _dd (not view) because it has a different lifecycle than a view. A single tab can have many views (SPA navigations), while the tab ID persists across them via sessionStorage. Placing it under view would be misleading since view.id changes per navigation. _dd is the right home as internal SDK metadata, alongside browser_sdk_version and sdk_name.

Behavior

Scenario	Behavior
New tab	Generates UUID, persists to sessionStorage
Page reload	Reuses existing UUID from sessionStorage
Tab duplication	Both tabs share the same ID (accepted tradeoff)
sessionStorage unavailable	In-memory UUID per page load

Test instructions

yarn test:unit --spec packages/core/src/domain/contexts/tabContext.spec.ts
yarn test:unit  # full suite passes
yarn typecheck
yarn lint

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 ✍️ ✅
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[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	169.44 KiB	169.67 KiB	+233 B	+0.13%	✅
Rum Profiler	4.29 KiB	4.29 KiB	0 B	0.00%	✅
Rum Recorder	24.61 KiB	24.61 KiB	0 B	0.00%	✅
Logs	56.72 KiB	56.94 KiB	+225 B	+0.39%	✅
Flagging	944 B	944 B	0 B	0.00%	✅
Rum Slim	126.29 KiB	126.51 KiB	+225 B	+0.17%	✅
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	0.0052	0.0039	-25.00%
RUM - add action	0.0186	0.0143	-23.12%
RUM - add error	0.0184	0.0137	-25.54%
RUM - add timing	0.0033	0.0028	-15.15%
RUM - start view	0.0152	0.0134	-11.84%
RUM - start/stop session replay recording	0.001	0.0007	-30.00%
Logs - log message	0.0181	0.0152	-16.02%

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	27.92 KiB	27.48 KiB	-455 B
RUM - add action	50.56 KiB	51.90 KiB	+1.34 KiB
RUM - add timing	26.35 KiB	27.43 KiB	+1.08 KiB
RUM - add error	56.85 KiB	54.98 KiB	-1.86 KiB
RUM - start/stop session replay recording	25.61 KiB	26.14 KiB	+552 B
RUM - start view	453.91 KiB	460.03 KiB	+6.11 KiB
Logs - log message	44.93 KiB	45.95 KiB	+1.02 KiB

🔗 RealWorld