humanspeak / svelte-virtual-list / 28624839498

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Commit Message

fix(virtual-list): count collapsed margins in item height measurement (#415)

* test(virtual-list): add failing spec for margins escaping height measurement (#412)

Fixture: 500 items, each a 100px border box with margin: 12px 0 that
collapses through the unstyled item wrappers, so real layout pitch is
112px while border-box measurement reads 100px. Estimate pinned to the
border box so margins are the only variable.

Two assertions, both failing pre-fix:
- cache-implied pitch (scrollHeight / itemCount) vs rendered layout
  pitch: off by exactly 12px (totalHeight ~6,000px short)
- one-pitch-per-step scroll sweep: tracked item jumps up to 24px at
  render-range boundaries (multiples of the lost margin)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(virtual-list): measure item pitch from layout offsets so margins can't escape (#412)

Item heights were measured as the wrapper's border box, which excludes
margins — and consumer margins (e.g. margin: 12px 0) collapse straight
through the component's unstyled item wrappers, permanently
under-counting every item with no ResizeObserver event to correct on.

measureItemPitch derives each rendered item's height from layout ground
truth instead: the wrapper's offsetTop delta to its next sibling, the
last closed by the items container's bottom edge (the container is
absolutely positioned — a BFC — so its auto height includes the last
child's bottom margin). Collapsed margins are included by construction,
whatever CSS consumers use. The ResizeObserver significance check now
compares pitch against the (pitch-holding) cache so margined items
don't churn as permanently dirty.

Margin-less consumers are numerically unaffected: offset-delta pitch
equals border-box height when nothing escapes. Falls back to the border
box when the element is not laid out (detached / non-browser tests).

Verified: 314 unit tests, svelte-check clean, full Playwright suite
green on chromium, firefox, and webkit.

Co-Authored... (continued)

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