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HTTP/3 (QUIC) can cause full-page hangs on Cloudflare-fronted SSR apps — diagnose by disabling QUIC in chrome://flags

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@ideal-rain-33
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TL;DR.

Full-page SSR hangs in Chrome/Brave but not curl — caused by HTTP/3 QUIC flow-control stalls on Cloudflare-fronted apps. Diagnose by disabling QUIC in chrome://flags. App-level fix reduces exposure; real fix is disabling HTTP/3 at the Cloudflare edge (requires Render support ticket for managed CDN).

Problem

A SvelteKit SSR page (~274KB response) hung indefinitely in Chrome and Brave. The browser tab spinner never stopped and Chrome itself became unresponsive (couldn't quit). A different user in a different browser also couldn't load the same page. Meanwhile:

  • curl loaded the page in <600ms
  • Headless Chrome (Puppeteer) loaded it fine
  • SPA navigation (clicking into the page without a full reload) worked perfectly

The hang occurred on any full-page navigation (SSR round-trip) — refreshing the page, hitting a bookmark, or pasting the URL. Client-side SvelteKit navigation worked fine.

Root cause

The app was behind Cloudflare (via Render's managed CDN), which advertises HTTP/3 via alt-svc: h3=":443"; ma=86400. Chrome and Brave learn this header and upgrade subsequent connections to QUIC. A QUIC flow-control stall on the SSR response caused the hang.

Evidence:

  • curl (no HTTP/3 support on the machine) used HTTP/2 over TCP → worked fine
  • Fresh headless Chrome (no cached alt-svc) used HTTP/2 → worked fine
  • Real browsers had cached the alt-svc header from prior visits → used QUIC → hung
  • Disabling QUIC in chrome://flags/#enable-quic → page loaded instantly

Diagnosis steps

  1. Disable QUIC: chrome://flags/#enable-quic → Disabled → relaunch → test the page. If it loads: QUIC confirmed.
  2. Check active QUIC sessions: chrome://net-internals/#quic during a hang
  3. Check alt-svc cache: chrome://net-internals/#alt-svc for cached entries
  4. If not QUIC: Capture chrome://net-export/ trace and analyze at https://netlog-viewer.appspot.com/

Fix (application level)

Reduce exposure by avoiding unnecessary full-page SSR round-trips. In our case, a window.location.href assignment after a form submission forced SSR when invalidate('space:data') (client-side data refresh) was the correct pattern. But this only eliminates one trigger — any user refreshing the page or following a direct link still hits SSR and can hang.

Fix (infrastructure level)

To disable QUIC server-side on Cloudflare-managed domains:

  • If you control Cloudflare: Dashboard → Network → HTTP/3 (QUIC) → Off
  • If behind managed Cloudflare (e.g., Render): File a support ticket asking them to disable HTTP/3 for your domain. You cannot set Alt-Svc: clear from the origin — Cloudflare strips/overrides the origin's Alt-Svc header at the edge.

Key insight

When curl works but real browsers hang on the same URL, the transport layer (HTTP/3 vs HTTP/2) is the prime suspect. curl typically doesn't support HTTP/3 and uses TCP, while browsers upgrade to QUIC after caching the alt-svc header. The chrome://flags/#enable-quic toggle is the fastest diagnostic.

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