Expose API to update size cache for VList/Virtualizer

[Horuse]

Is your feature request related to a problem? Please describe.

With variable-height items in <VList> / <Virtualizer>, a drag-and-drop reorder produces a one-frame flicker (~1-2px shift, sometimes more) before the layout settles.

virtua's size cache is keyed by index (Cache._sizes[i] in src/core/cache.ts), so after reordering items in the data array the same index points to a different item with a different real height.

1. User updates data, so the same index now references a different item.
2. virtua renders with the stale cached height (_sizes[index]).
3. ResizeObserver delivers new sizes, ACTION_ITEM_RESIZE in
   src/core/store.ts computes a jump and bumps stateVersion.
4. The effect tied to stateVersion calls scroller.$fixScrollJump(),
   writing scrollTop += jump.
5. Browser already painted one frame with stale heights, hence the flicker.

Lists with uniform heights don't reproduce this because cached values remain valid after permutation. The repo's existing dnd-kit story uses uniform heights, which is why it doesn't surface the issue.

Describe the solution you'd like

Some way to update the index/size mapping from user code before the reorder lands, so that the subsequent ResizeObserver callback finds the cache already consistent and skips its correction pass.

for example:

vlist.setItemSizes([[0, h0], [1, h1], ...]);

Describe alternatives you've considered

• requestAnimationFrame + scrollBy(delta). Always lands at least
  one frame after virtua's own $fixScrollJump, leaving sub-pixel jitter.
• scrollTo(getScrollOffset()) save/restore. Produces an extra scroll
  event, perceived as double jitter.

Additional context

Example (virtua + dnd-kit): https://stackblitz.com/edit/vitejs-vite-4xibpcqs?file=src%2FApp.tsx

[inokawa]

It seems like an issue specific to dnd-kit. As mentioned in #604 (comment), dnd-kit was completely rewritten. Basically I don't have a plan to add an API for outdated dnd-kit.

#852

[Horuse]

The old dnd-kit called arrayMove once in onDragEnd. The new dnd-kit calls
'move(items, event)' in each onDragOver, splitting one large rearrangement
into many small ones. So new version of dnd-kit actually doesn't have this issue, but there are many other libraries, or simply different ways of using it, where the same issue will occur

Here's a minimal example without dnd-kit at all - just a button that scrolls
to an item and another that moves item index 0 to 500. Same flicker:
https://stackblitz.com/edit/vitejs-vite-yvk56mnt?file=src%2FApp.tsx

2026-05-04.01.33.23.mov