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CSS Module Styles Lost After Client Navigation When Page CSS Gains `Data-N-P` After ISR Revalidation

In Pages Router, when a component with a CSS module is statically imported on one route and dynamically imported on another, after ISR revalidation of the static route, client navigation to the dynamic route removes the shared stylesheet and does not re-add it, resulting in missing styles.

highConfidence 85%Next.js Affected V15.5.18 Affected V16.3.0-Canary.103

Origin Analysis

Next.js client-side router removes page-level stylesheets (marked with `data-n-p`) on navigation, but does not account for stylesheets that are also present in the `dynamic-css-manifest.json`. This manifest was introduced in PR #72959 for dynamic CSS handling. When a CSS module is shared between a static import and a dynamic import, the same CSS file is both a page-level style and a dynamic dependency. After ISR regenerates the static page, the stylesheet link gains `data-n-p`. On client navigation to the dynamic route, the router removes any `data-n-p` link, eliminating the shared CSS. The dynamic import loader then assumes the CSS is already loaded (by href) and does not re-inject it, leaving the component unstyled.

1. Clone https://github.com/hshahdramafever/nextjs-bug-demo 2. Run `npm install && npm run build && npm start` 3. Wait about 5 seconds (or until the home page's `revalidate: 5` expires) after build finishes. 4. Open http://localhost:3000/ and click 'Go to second page →' — initially styled correctly. 5. Navigate back to home and reload the page (full document load). 6. Click 'Go to second page →' again. 7. Observe that the box has CSS module class names but no styles; DevTools shows the stylesheet link is removed and not re-fetched.

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

This workaround may cause duplicate stylesheet loading if the original link is not removed or if multiple route changes occur rapidly; it also introduces an extra network request for the manifest on every route change. It does not fix the underlying router logic and may behave inconsistently across Next.js versions. Users should test thoroughly and consider upgrading to an official fix when available. Rollback is straightforward: remove the code from `_app.js`.