What

When a component imports and overrides styles from another component, the CSS cascade order could be violated in production builds. For example:

CardWrapper.module.css (sets background: purple)
  └── imports Card.module.css (sets background: white)

The expected behavior is that CardWrapper's purple background overrides Card's white background. However, in production builds, the shared chunk containing Card's CSS could be emitted before CardWrapper's CSS, causing the wrong color to appear.

Root Cause

The style chunking algorithm groups CSS modules into shared batches to reduce HTTP requests. The bug was that shared batches were emitted at the position of their first member in the topological order, rather than their last member.

Consider this dependency graph with post-order traversal [C, B, A] where A depends on B:

A → B
C

If C and A are grouped into a shared batch, the old code would emit that batch when it first saw C - before B. This violates the constraint that A must come after B.

The Fix

style_production.rs: Track how many items remain for each shared batch. Only emit a batch when all its members have been seen (i.e., at the position of its last member).

style_groups.rs:

• Use FxIndexMap for chunk_group_indices to ensure deterministic iteration order
• Minor cleanups (use FxHashSet directly instead of collecting to Vec)
• Fix how module_dependents were computed to be more accurate. The prior approach could infer more dependencies than there actually were

Fixes #89523
Fixes #83941
Fixes #86704
Fixes #88604
Fixes #87321
Fixes #86459