Expose an API to cancel active scroll reconciliation

Describe the use case

A virtualizer may have a long-distance scrollToIndex reconciliation in progress while the user begins a genuine wheel, touch, pointer, or keyboard scroll. The application must immediately hand viewport ownership back to the user.

In @tanstack/virtual-core 3.17.8, reconcileScroll can continue correcting toward the old target for up to the five-second safety limit. There is no public way to cancel it. The only effective application workaround is a private-field write:

;(virtualizer as unknown as { scrollState: unknown }).scrollState = null

That works, but is intentionally outside the public contract and can silently break on upgrade.

Requested API

A public method such as:

virtualizer.cancelScrollTo()

It should cancel the active scrollToIndex / scrollToOffset reconciliation target without changing the current DOM offset. This lets a user gesture take ownership immediately while preserving normal subsequent measurement compensation.

Why this belongs in core

• Core owns the reconciliation state and its lifecycle.
• Consumers cannot reliably infer or cancel every pending internal frame/timer.
• User input cancellation is needed by chat timelines and other dynamic, long-distance virtual navigation surfaces.

Version

@tanstack/virtual-core 3.17.8; the same private-only state is present on current main.

Start in the @tanstack/virtual-core implementation around reconcileScroll and the scrollState used by scrollToIndex and scrollToOffset. Trace how the active reconciliation target and its safety limit are managed, then inspect the existing core tests if available. Done means a public cancelScrollTo API stops the active target without changing the current DOM offset and preserves later measurement compensation.