`useVirtualizer({paddingEnd: 800})` creates overscroll issue with `scrollToIndex(last)`

リンクされた最小再現から始め、useVirtualizer と scrollToIndex のエントリポイントを追跡し、paddingEnd と last-index の挙動に注目してください。提供されているボタンで問題を再現し、最後のインデックスまでスクロールしたときに、コンテナを過剰にスクロールさせずに項目が完全に表示されたままになることを確認してください。

Describe the bug

The paddingEnd option for the useVirtualizer callback is not correctly taken into account when using rowVirtualizer .scrollToIndex(last). Instead the function call scrolls to the end of the scroll container. For every other index this works as intended.

const rowVirtualizer = useVirtualizer({
  count: 100,
  getScrollElement: () => parentRef.current,
  estimateSize: () => 35,
  overscan: 5,
  paddingEnd: 800
})

// ...

<button onClick={() => {
  rowVirtualizer.scrollToIndex(98)
  }}>
  Scroll to 98 (works)
</button>
<button onClick={() => {
  rowVirtualizer.scrollToIndex(99)
  }}>
  Scroll to 99 (overscrolls)
</button>

Your minimal, reproducible example

https://github.com/Freymaurer/tanstack-virtual-padding-overscroll-bug-example

Steps to reproduce

1. Clone and npm i -> npm run dev the example (fresh vite-react-ts with tanstack virtual dependency) -> open the link printed to console
2. Click the buttons i added on top of the view

• "Back to Start" uses scrollToIndex(0)
• "Scroll to 98 (works)" scrolls to 98, so not last index
• "Scroll to 99 (overscrolls)" scrolls to the last index

3. Click on "Back to Start", then "Scroll to 98 (works)". It scrolls with pretty good alignment. The field is shown on the bottom of the scrollContainer
4. Click on "Back to Start", then "Scroll to 99 (overscrolls)". It overscrolls, the element is not visible on screen. 🐛 I think this is due the library calling more or less "scrollToEnd" instead of "scrollTo" in this scenario.

Expected behavior

I expected the behavior to be consistent. The element at last index should be fully visible after calling the scroll function, instead of scrolling to the scroll container end

How often does this bug happen?

Every time

Screenshots or Videos

After clicking on "Scroll to 98 (works)"

After clicking on "Scroll to 99 (overscrolls)"

Platform

OS: Windows 11
Browser: Tested on Chrome, Edge, Firefox, Opera

tanstack-virtual version

v3.14.9

TypeScript version

v6.0.3

Additional context

No response

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• I understand that if my bug cannot be reliable reproduced in a debuggable environment, it will probably not be fixed and this issue may even be closed.