🐛 Prevent duplicate Next.js RUM views from discarded renders

[BeltranBulbarellaDD]

Motivation

Fixes #4931, where DatadogAppRouter and DatadogPagesRouter could call startNextjsView() during a render that React later discarded. Because the render-phase useRef guard is recreated for discarded fibers, those calls could create duplicate RUM views.

Changes

• Move the shared view-start logic into useStartNextjsView, which calls startNextjsView() from useLayoutEffect after the render commits.
• Reuse the hook in both App Router and Pages Router integrations while preserving route-pattern view names and concrete-path navigation detection.
• Wait for the Pages Router to be ready before starting the initial view.
• Add unit tests for Strict Mode, route changes, and router readiness.
• Add an end-to-end regression test covering a discarded App Router render.

Test instructions

• Run the Next.js router unit tests.
• Run the Next.js plugin E2E scenario, including should not create a view for a discarded App Router render.

Checklist

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

[datadog-datadog-prod-us1]

 

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ℹ️ Info

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[lierniel]

Question:
Why not just useEffect() ? We're blocking the render by using useLayoutEffect(), which is not that I would expect from a monitoring tool

[BeltranBulbarellaDD]

The thing is that useLayoutEffect runs before paint and just by doing this fix we are going to be starting views later that what we would like to. useEffect would be just too late to start a view.