fix(solid-virtual): defer non-sync onChange updates to a microtask

[moreal]

🎯 Changes

The Solid adapter applies every onChange immediately, including the ones virtual-core reports with sync: false. Those come from resizeItem() / measure(), which a row triggers from its own ref — while <For> is still iterating the virtual item store. reconcile then mutates that array in place, mapArray reads past the shortened array, and the row callback receives undefined:

TypeError: Cannot read properties of undefined (reading 'index')
 ❯ mapper solid-js/dist/dev.js:1229

virtual-core already distinguishes the two cases, and react-virtual acts on it (flushSync(rerender) when sync, plain rerender() otherwise). The Solid adapter received sync and only forwarded it to the user callback.

This applies the same distinction: scroll-driven updates stay synchronous, size changes are flushed in a microtask — after the current pass, before paint. Option changes (count, scrollMargin, …) also stay synchronous, since they run in Solid's reactive update phase rather than mid-render.

Behavior change: reading getVirtualItems() / getTotalSize() in the same tick as resizeItem() or measure() now returns the previous value. Reading before the re-render gives the same result in React, so this makes the adapters consistent. One existing test asserted the old timing and was updated.

tests/measure-during-render.test.tsx renders a <For> list with a measuring row ref and fails with the exception above without this change. The ref writes data-index before measuring because Solid emits _$use(ref, el) ahead of _$setAttribute (#930) — that workaround is what Solid consumers have to write today for measurement to run at all.

Related: #1216 and #790 touch the same onChange block. Neither fixes this — I applied #1216's change (dropping reconcile) locally and the reproduction still throws, because the problem is when the store is written, not how it is diffed.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

[coderabbitai]

Important

Draft PR not reviewed

Draft PRs are not automatically reviewed by default.

• Trigger a manual review

To automatically review draft PRs, update your CodeRabbit configuration:

reviews:
  auto_review:
    drafts: true

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}