laneAssignmentMode 'measured': is it intended that a row measured at exactly its estimate can change lanes? #1379

Describe the bug

With lanes > 1 and laneAssignmentMode: 'measured', a row whose first measurement exactly equals estimateSize doesn't get its lane cached, because resizeItem records a measurement only when the size changes. When a row above it grows later, that row is placed again and can move to another lane. A row measured at any other size keeps its lane.

Your minimal, reproducible example

import { Virtualizer } from '@tanstack/virtual-core'

const measureThenGrow = (height: number) => {
  const virtualizer = new Virtualizer({
    count: 8,
    lanes: 2,
    estimateSize: () => 100,
    laneAssignmentMode: 'measured',
    getScrollElement: () => null,
    scrollToFn: () => {},
    observeElementRect: () => {},
    observeElementOffset: () => {},
  })
  // getMeasurements is private; tests/index.test.ts reads it the same way.
  const lanes = () =>
    virtualizer['getMeasurements']().map((m) => m.lane).join(' ')

  lanes() // Build the measurements first; resizeItem needs them.
  for (let i = 0; i < 8; i++) virtualizer.resizeItem(i, height)
  const measured = lanes()
  virtualizer.resizeItem(1, 500)
  return { measured, grown: lanes() }
}

console.log(measureThenGrow(100))
// { measured: '0 1 0 1 0 1 0 1', grown: '0 1 0 0 0 0 1 0' }
console.log(measureThenGrow(101))
// { measured: '0 1 0 1 0 1 0 1', grown: '0 1 0 1 0 1 0 1' }

Steps to reproduce

1. Run the example above.
2. At 100, row 1 growing moves rows 3, 5, 6 and 7 to another lane.
3. At 101, every row keeps its lane.

Expected behavior

I can't tell which behavior is intended, so I'd like to ask before going further.

• If this is not intended, a row measured at exactly its estimate should keep its lane like any other measured row, since the docs say lanes are cached and remain stable after the initial measurement. fix(virtual-core): keep the lane of a row measured at its estimate #1377 is a draft of that fix.
• If this is intended, for example so that a placeholder sized like the estimate can rebalance once its content loads, there is nothing to fix and I'll close fix(virtual-core): keep the lane of a row measured at its estimate #1377.

Is this behavior intended?

How often does this bug happen?

Every time

Platform

Any. This is in virtual-core.

tanstack-virtual version

3.19.11

TypeScript version

n/a

Terms & Code of Conduct

• I agree to follow this project's Code of Conduct
• I understand that if my bug cannot be reliably reproduced in a debuggable environment, it will probably not be fixed and this issue may even be closed.