Introduction

If your website is built using a JavaScript framework or follows a Single‑Page Application (SPA) model—think React, Vue, or Angular—you’ve likely faced a frustrating reality: Core Web Vitals like LCP, CLS, and INP often don’t get tracked properly. Traditional tools like Lighthouse, CrUX, or RUM scripts can miss soft navigations—those in-page updates that happen without a full reload.

The Chrome 139 Origin Trial

Chrome is addressing this blind spot through the Soft Navigations API available in Chrome 139. When enabled, this feature:

• Detects “soft”

  •  navigations where the URL changes and DOM re‑renders occur.
  • Triggers performance measurement for Largest Contentful Paint (LCP) after the soft navigation.

• Adds navigationId

  • so tracking systems can link UX metrics to each navigation event.
  • Extends CLS and INP tracking across soft navigations to ensure responsiveness and stability are measured per interaction—even within SPAs.

SEO & UX Impact

• SPAs no longer remain “invisible” to Core Web Vitals reporting.
• Better visibility into real user experiences means more accurate performance audits and SEO performance tracking.
• Early adopters can proactively identify and fix UX bottlenecks before Google integrates these metrics publicly (e.g. CrUX dashboards).

How You Can Test It

• In Chrome 139, enable flag: chrome://flags/#soft-navigation-heuristics

  • Join the origin trial by embedding a meta‑tag or HTTP header token on your site.

• Optionally, enable Advanced Paint Attribution for deeper measurement fidelity.

  • Ensure your analytics or RUM provider reads new fields like navigationId and interaction-contentful-paint

• Document edge cases: auto-redirects, replaceState()

  • usage, or custom navigational logic may not be caught—test thoroughly using real flows.

Caveats to Consider

• It’s experimental—Chrome only, and only versions 139+.
• Other browsers (Safari, Firefox) won’t support this yet.
• Your performance tooling must adapt to ingest new metrics; older dashboards might ignore them

Quick Recap Table

Feature	Before	With Soft Navigations API
SPA Traffic Tracking	Missed soft navigations	Measures LCP, CLS, INP in-page
Navigation Tracking	Page reload only	Navigation ID enables per-event tracking
RUM Data Granularity	Coarse metrics	Fine-grained, per-navigation UX metrics

Why This Is a Game‑Changer

This Chrome trial fills a critical gap. SPA-based websites can now begin closing the visibility gap—getting genuine real‑user feedback on performance. This positions you ahead of the curve when Google eventually updates its public dashboards like CrUX to include soft navigation metrics.

If you’re responsible for performance or SEO on an SPA, this is your moment to act—and gain that SEO competitive edge.