Storefront and experience

A Horizon 4.2.0 source walkthrough of what happens when a shopper switches variants and clicks add to cart before the section request returns: a ProductSelectEvent carrying a promise, a generation counter, a queue of intent snapshots, pure-function variant resolution, quantity renormalized for the new variant and a single batched POST, plus debatable points such as animating on enqueue.

A set of techniques from the Horizon 4.2.0 source for avoiding forced synchronous layout: measuring header heights before first paint only when needed, getting sizes from IntersectionObserver, batching reads and writes in the variant picker, a ResizeNotifier that skips the first callback, a Scheduler that waits for transitions, yielding to the main thread, and composing the fly-to-cart arc with CSS animations.

A Horizon 4.2.0 source walkthrough of how contrast colors are derived on the server when a block has a custom background: the palette's lightest and darkest colors, contrast-override's three-step text fallback and border fallback, brightness-based opacity tokens, hover color derivation, skipping contrast over background media, and issues such as the hard-coded brightness threshold and a preset description that doesn't match the code.

A Horizon 4.2.0 source walkthrough of cross-document View Transitions in a multi-page theme: a settings-driven opt-in, render blocking until main content with a 1.8 second cap, releasing it when no transition can happen, skipping on user input, switching animations by type and passing the type across pages, plus the in-app WebView blocklist and in-page transitions.

A Horizon 4.2.0 source walkthrough of the framework-free Component base class: automatic ref collection and ownership, global event delegation through on: attributes, the custom element upgrade race, declarative Shadow DOM hydration, and how updatedCallback is meant to work with morph, plus a doubt about whether it fires.

A Horizon 4.2.0 source walkthrough of morph's hydration mode: matching only by data-hydration-key, the no-insert, no-remove contract, three use cases in product recommendations, the cart drawer and the header, the header branch that leaves featured products out of the first render, and the limits of this two-pass rendering.

A Horizon 4.2.0 source walkthrough of SectionRenderer and the in-house morph: per-section cancellation and result reuse, sorted cache keys, first-paint caching and limited request deduplication, plus morph's form-state sync, mid-edit protection, runtime state preservation, skip switches and a few debatable couplings.