feat: add Fetch & XHR Timeline DevTools Override snippet

[nucliweb]

Summary

• Adds a two-part DevTools Override snippet that intercepts all fetch() and XHR calls from page initialization and correlates them with LCP timing
• Inject snippet patches window.fetch and XHR.prototype before any other script runs, capturing calls invisible to the Network panel (cross-origin without Timing-Allow-Origin)
• Read snippet generates a structured console report covering four diagnostic cases:
  • Calls before LCP — critical path candidates with deferral/caching recommendation
  • Bootstrap errors — failed calls before LCP that silently add latency
  • Auth-related calls — detected by URL pattern (/auth, /login, /token, /session, /oauth, /me, /whoami, /identity, authIndexType query param)
  • Complete network inventory — all calls with Auth column and cross-origin visibility note

Test plan

• Enable DevTools Overrides on a target page and add the inject snippet inside <script> before </head>
• Reload the page, then run the read snippet in the console
• Verify LCP timing appears and Before LCP column shows ⚠️/no correctly
• Verify auth-related calls are flagged with 🔑
• Verify bootstrap errors section appears when calls fail before LCP

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
webperf-snippets	Ready	Preview, Comment	Apr 20, 2026 3:20pm
webperf-snippets-u6am	Ready	Preview, Comment	Apr 20, 2026 3:20pm