fix(scroller): don't throw from scroll operations before an element is set

[schwarz9791]

Summary

Scroller threw missing element when it was operated before an element had been set. @fastkit/vue-scroller reaches this: useScrollerControl({ el: 'self' }) (and VScroller) only sets the element in onMounted, so while a <Suspense> boundary is still pending, the control exists but its Scroller has no element yet.

Scroll operations

by(), to(), toElement(), toSide(), toTop() and the other side/corner methods built their options through _checkElement(), which threw. All of them go through one private method, which now returns a result that does nothing and resolves immediately when there is no element (no onCancel / onDone). The options are only built after that check.

Scroll stoppers

pushScrollStopper() / removeScrollStopper() threw the same way via _updateScrollable(). It now skips when there is no element; the stopper stays recorded and _setup() applies it once the element is set. On a destroyed instance pushScrollStopper() ignores the stopper, so scrollEnabled stays as it is.

Since destroy() drops the element, these calls after destroy() also do nothing instead of throwing. This matches the existing setters (scrollTop etc.), which already return when there is no element.

Tests

• packages/scroller/src/__tests__/scroll-without-element.client.spec.ts
  • every scroll operation on a Scroller without an element: does not throw, promise resolves, plans is { x: 0, y: 0 }
  • a stopper pushed before the element is set locks scrolling once the element is set, and removing it unlocks it
  • a scroll operation after destroy(): promise resolves, plans is { x: 0, y: 0 }
  • pushScrollStopper() after destroy(): scrollEnabled stays true

All of them fail before the fix and pass after it.

Release

• @fastkit/scroller patch
• @fastkit/vue-scroller patch, so its published range becomes ^1.0.2 and updating it brings the fix along (see "Delivering a fix in a dependency to its dependents" in docs/release.md)

🤖 Generated with Claude Code

https://claude.ai/code/session_015hoZxV4JFvpJpTzs5ZxqH6

[changeset-bot]

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[schwarz9791]

コードレビューで見つかった指摘を検証し、残ったものをインラインでまとめました。ブロッカーはありません。

[schwarz9791]

76a6dc9 を再レビューしました。指摘への対応はどれも妥当で、追加の指摘はありません。scroller の typecheck / lint / test（16 件）が通ることも手元で確認しました。

[nkenji09]

この PR が入れた不具合は見つかりませんでした。PR の前からある不具合を 5 件、参考としてインラインに添えています（別対応で構いません）。