# Proposal

## Why

The mall home page shows the category menu as a hover dropdown that auto-hides once the shopper scrolls 200px, and the hero carousel spans the full 1200px grid. Referencing tigshop's B2B2C storefront, the category tree should be a permanent left rail beside the banner so shoppers always see the catalog structure without hovering or scrolling back up. Additionally, the header's scroll-collapse hides the logo/search/cart bar on scroll, which hurts usability.

## What Changes

- Home page hero becomes a flex row: a pinned 240px category sidebar (reusing `ShellCategoryMenu`) left of the carousel; both are 450px tall and in-flow, not overlaid.
- Carousel narrows from 1200px to ~950px; banner images (1200×450) are center-cropped left/right at fixed 450px height.
- Hovering a top-level category in the pinned sidebar expands the white mega-menu panel to the right, overlaying the carousel (same content as today's flyout).
- Non-home pages keep the hover dropdown under the nav "全部商品分类" entry, unchanged.
- **BREAKING** (visual): the header no longer auto-hides on scroll — the `collapsed`/`scrollY > 200` logic is removed entirely, so the logo/search/cart bar and category menu never auto-hide.

## Capabilities

### New Capabilities

(none)

### Modified Capabilities

- `frontend-mall`: the "Mock PC home page" requirement gains a pinned category sidebar beside the cropped carousel; the "B2B2C mall-style PC storefront shell" requirement changes the category mega-menu to hover-only on non-home pages and removes scroll-based header auto-hide.

## Non-goals

- No changes to mock category data, banner assets, or the search/category filtering logic.
- No changes to apps/shop-admin, apps/admin, the shared package, or the backend.
- No right-side user rail, no extra promo tiles in the hero (options B/C/D from the visual proposal were rejected; see `docs/visual-proposal/home-hero-options.html`).