AI & Agent Dev Bug Sandbox Back to Radar

Pages Router: CSS Shared By A Page And A Next/Dynamic Component Is Removed After Client Navigation

When a CSS file is imported by both a page and a component loaded with next/dynamic and kept mounted via _app, the stylesheet link is incorrectly marked with data-n-p and removed during client-side navigation. This happens because the dynamic CSS manifest is not loaded correctly in the route module and not passed to _document.

highConfidence 95%Next.js Affected V15.5.25 Affected V16.4.0-Canary.55

Origin Analysis

In Next.js 15.4+ Pages Router refactor, the route module loads 'dynamic-css-manifest' without the .json extension and with handleMissing: true, which silently returns an empty object. Additionally, pages-handler does not pass the dynamicCssManifest to renderOpts, so Head.getCssLinks receives an empty set and marks all route stylesheets with data-n-p, causing them to be removed on client navigation.

1. Clone https://github.com/ithinkdancan/next-dynamic-css-repro\n2. npm install && npm run build && npm start\n3. Open http://localhost:3100/ - both red boxes are styled\n4. Click 'Go to /other' for client-side navigation\n5. Observe that the persistent box rendered from _app via next/dynamic loses its red background

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

The fix assumes the dynamic-css-manifest.json file exists at runtime; with handleMissing: true it will not crash if missing but may produce an empty manifest. In concurrent builds or custom server setups, ensure the manifest path and filename remain consistent. If deploying alongside older Next.js versions, the changed property may not be recognized; rollback to the provided _document prototype workaround if integration issues arise. Test with both webpack and turbopack builds, and verify that no additional stylesheets are inadvertently retained after navigation.