fix(virtual-core): use getMaxScrollOffset() - paddingEnd for scrollToIndex(last) end-align - #1276

Summary

Fixes scrollToIndex(last, { align: 'end' }) so it scrolls to the virtual max offset (content end) rather than the raw DOM max scroll offset (content + paddingEnd) when paddingEnd > 0.

Problem

When paddingEnd is set on a virtualizer, getOffsetForIndex(last, 'end') returned getMaxScrollOffset() = scrollHeight - clientHeight. Since scrollHeight includes paddingEnd, the returned offset overshot the rendered end of the last item by exactly paddingEnd pixels. This made scrollToIndex(count - 1, { align: 'end' }) scroll past the last item.

Solution

Subtract paddingEnd from getMaxScrollOffset():

if (align === 'end' && index === this.options.count - 1) {
  return [
    Math.max(this.getMaxScrollOffset() - this.options.paddingEnd, 0),
    align,
  ] as const
}

This gives (content - clientHeight) — the correct virtual max offset that keeps the last item flush with the bottom of the viewport.

Why getMaxScrollOffset() and not getTotalSize()?

Using getMaxScrollOffset() directly (rather than getTotalSize() - paddingEnd - getSize()) preserves the lane-max behavior added in #1105 (#1001). In multi-lane layouts where the last item lives in a shorter lane, getMaxScrollOffset() still absorbs DOM extras (borders, padding, unmeasured dynamic items) that aren't in our measurements. Targeting item.end would regress #1001 by scrolling the last item above the viewport top.

Changes

• src/index.ts: Subtract paddingEnd from getMaxScrollOffset() for last-item end alignment
• tests/index.test.ts: 3 regression tests for useVirtualizer({paddingEnd: 800}) creates overscroll issue with scrollToIndex(last) #1257 (paddingEnd overshoot) and fix(virtual-core): scrollToIndex(last) overshoots when paddingEnd > 0 #1263 (lane-max preservation)
• changeset: patch for @tanstack/virtual-core

Testing

All new tests pass. Existing tests (including the #1258 clamped-growth suite) are unaffected.

Closes #1263
Closes #1257

Summary by CodeRabbit

• Bug Fixes
  • Fixed end-aligned scrolling to the last item so it respects scrollPaddingEnd instead of overshooting due to trailing padding.
  • Preserved the maximum scroll position for multi-lane layouts.
  • scrollToEnd() continues to scroll to the bottom, including trailing padding.