RSC manifest omits chunks when CSS files precede JS in chunk.files Set

[AbanoubGhadban]

Bug

The ReactFlightWebpackPlugin (vendored from react-server-dom-webpack) silently drops chunks from the client manifest (react-client-manifest.json) when a chunk contains both .css and .js files and the .css file is iterated first in the chunk's files Set.

Root Cause

In the processAssets hook, the plugin iterates c.files using a for...of loop inside a forEach callback:

chunkGroup.chunks.forEach(function (c) {
    for (const file of c.files) {
        if (!file.endsWith('.js')) break;       // ← BUG: exits entire loop
        if (file.endsWith('.hot-update.js')) break;
        chunks.push(c.id, file);
        break;
    }
});

The first break (compiled from return in the React source) is intended to skip non-JS files and try the next file. But break exits the for loop entirely (and return in the source exits the forEach callback), so when a .css file appears before the .js file in the Set iteration order, the .js file is never examined. The chunk is completely omitted from the manifest.

Impact

• The client manifest's chunks array is empty for affected modules
• The RSC Flight runtime can't locate client components → RSC fails silently or errors at hydration
• The bug is non-deterministic: it depends on Set insertion order, which varies across webpack versions and environments
• Triggered by mini-css-extract-plugin or any loader that produces .css chunk files alongside .js

Fix

Change break to continue (compiled) / return to continue (source) for the two skip conditions:

// Fixed:
for (const file of c.files) {
    if (!file.endsWith('.js')) continue;           // skip non-JS, try next file
    if (file.endsWith('.hot-update.js')) continue;  // skip HMR, try next file
    chunks.push(c.id, file);
    break;                                          // found JS file, done
}

Fix Commit

The fix has been applied to the React fork:
AbanoubGhadban/react@8b6dec4

Branch: fix/flight-plugin-css-chunk-skip (based on rsc-patches/v19.0.3)

Upstream

This is an upstream bug in facebook/react, introduced in commit 701ac2e5 (PR #27314, Sep 2023) and still unfixed on main as of March 2026. Next.js is unaffected because they wrote their own plugin using nested forEach (where return naturally acts as continue).