feat(calendar): infinite horizontal day scroll in week view

[h4yfans]

What

Replace the fixed 7-day CSS grid in Calendar Week view with a continuously-scrollable horizontal day strip. Trackpad horizontal swipes (and Shift + mouse wheel) pan the visible range day-by-day; the toolbar month/year label updates live as columns slide. Prev/Next/Today buttons smooth-scroll to their target and share one source of truth with the gesture scroll.

Why

The old week view was a dead-end pager — seven columns, click Next to advance seven days. On a trackpad-centric macOS app users expect to scroll horizontally through time the same way they scroll vertically through hours. "Infinite" horizontal scroll also unifies the gesture + button navigation paths so the header label, fetch range, and scroll position never desync.

How

Core state shift. anchorDate is no longer the primary state — the scroll offset in pixels is. A 2020-01-01 day-index epoch keeps virtualization math simple (all dates become non-negative integer column indices). visibleDayStart = floor(scrollLeft / columnWidth); header label and fetch range derive from it.

Virtualization. @tanstack/react-virtual with horizontal: true renders only the visible columns + overscan over a logically 100-year-wide strip. Column width is (containerWidth − gutterWidth) / 7 tracked via ResizeObserver.

Layout. Mirrored scroll containers — the body owns the 2D scroll, a header strip (overflow-hidden) mirrors its scrollLeft, and the time gutter (overflow-hidden) mirrors its scrollTop. Simpler than sticky-top + sticky-left stacking in a single 2D container and keeps useScrollToCurrentTime working unchanged.

Flicker + scroll-back bug fixes. Two subtle issues surfaced during review:

1. Hook's scroll listener had onVisibleDayStartChange in its useEffect deps, and the shell passed an inline arrow — every parent re-render unregistered + re-added the listener, losing scroll events mid-gesture → flicker. Fixed by storing the callback in a ref so the effect deps are just [columnWidth].
2. The scroll handler was calling onVisibleDayStartChange inside a setVisibleDayStart updater function. That breaks React 18 auto-batching — the hook's visibleDayStart update and the page's setAnchorDate could commit in separate renders, and in the intermediate render the anchor-sync effect saw a mismatch and smooth-scrolled back to today. Fixed by calling both as siblings in the same native event handler + adding a lastEmittedAnchorRef in the view so self-initiated anchor updates don't re-trigger the anchor-sync effect.

Data. Week view fetch range widened to [weekStart − 7, weekStart + 14) so the React Query cache keeps ±1 week around the visible range loaded. Single expanded query is simpler than three parallel queries and the cache behavior is the same in practice.

Marquee. useTimeGridMarquee now accepts an optional getColumnElement resolver — the virtualized grid uses a data-day-index attribute lookup so drag-select and the quick-create popover anchor correctly against absolute day indices.

Scrollbar. Body scrollbar hidden via the existing .scrollbar-none utility — functionality unchanged.

Type

• feat — new feature

Test plan

• Unit tests added/updated — 6 new tests for dayIndexFromDate / dateFromDayIndex round-trip, DST boundary crossing, negative-index support
• Integration tests added/updated
• E2E tests added — calendar-week-scroll.e2e.ts covers scroll right without snap-back, scroll left from a forward position, Today button after scrolling far, and Prev/Next round-trip
• Manual testing — trackpad horizontal swipe, Shift + mouse wheel, Prev/Next/Today buttons, window resize stability

Verification run:

• pnpm typecheck:web clean
• pnpm lint 0 errors (all warnings pre-existing in unrelated files)
• pnpm test 6918 passed, 1 skipped
• E2E suite needs a fresh electron-vite build before running (per CLAUDE.md gotchas) — not executed in CI-blocking form from this worktree

Checklist

• Self-reviewed the diff
• No hardcoded secrets or credentials
• Files stay under ~500 LOC
• Follows immutable data patterns