Show HN: Forked styled-components with optimizations (40% faster for Linear)

Hey HN! We forked styled-components after it entered maintenance mode because our production apps (and many others) can't migrate overnight.

Backstory: We submitted [PR #4332](https://github.com/styled-components/styled-components/pull/...) to styled-components in July 2024 with React 18 optimizations. When maintenance mode was announced, we turned that PR into this fork.

What we fixed: - Added React 18's useInsertionEffect - Rewrote streaming SSR for React 19 - Replaced ES5 output with modern JS - Optimized array operations with native flatMap - Fixed Next.js App Router to work without 50+ lines of boilerplate

Linear tested it and saw 40% faster initial renders with zero code changes.

How to try it: npm install @sanity/styled-components@npm:styled-components

Or for React 19: npm install @sanity/css-in-js@npm:styled-components

Benchmark tool to test yourself: https://css-in-js-benchmarks.sanity.dev/

We named it "last-resort" because that's what it is. We're not trying to maintain styled-components long-term - we're actually migrating to vanilla-extract ourselves. This is just a performance bridge while teams migrate properly.

The React team recommends moving away from runtime CSS injection. We agree. But migrations take time, and production apps need to ship today.