Upgrading Checkout Blocks app to Polaris web components

Checkout Blocks’ UI extensions render on a third of all customized checkouts and were built on React and Shopify's legacy Remote UI bridge, running on the soon-to-be-sunset 2025-07 API version. Shopify is moving its entire UI extension surface to remote-dom with Polaris web components, so staying on the old path was no longer an option. We upgraded all five extensions to 2026-01 running on remote-dom, with Preact and Polaris web components, and rewrote the codebase to TypeScript. This is how we did it, and what we learned along the way. Checkout is an extremely performance-sensitive surface. Every extension adds buyer-facing latency: code that downloads, parses, and executes in the buyer's browser at the moment they're deciding whether or not to buy. Polaris web components and remote-dom cut that overhead directly: smaller bundles, faster page loads, and ultimately a faster checkout experience for the buyer. Starting October 1, 2026, app deployments will be blocked when they include extension versions earlier than 2026-01. Now is a great time to upgrade from legacy APIs and capture the associated performance gains. We set out to make our extensions leaner and faster, and to move a big chunk of checkout traffic off the legacy React…