anchorTo: 'end' does not keep the reading position when only paddingStart changes #1366

import { expect, test, vi } from 'vitest'
import { Virtualizer } from '../src/index'

// 20 items of 100px, a 300px viewport, anchorTo: 'end'. Content above the
// items (a "Load earlier messages" header) lives in paddingStart.
function create(paddingStart: number, offset: number) {
  let keys = Array.from({ length: 20 }, (_, i) => `m${i}`)
  let padding = paddingStart
  const scrollToFn = vi.fn()
  const scrollElement = {
    scrollTop: offset,
    scrollLeft: 0,
    scrollHeight: padding + keys.length * 100,
    scrollWidth: 1000,
    clientHeight: 300,
    clientWidth: 400,
    offsetHeight: 300,
    ownerDocument: {
      defaultView: {
        requestAnimationFrame: () => 1,
        cancelAnimationFrame: vi.fn(),
        performance: { now: () => Date.now() },
        ResizeObserver: vi.fn(function () {
          return { observe: vi.fn(), unobserve: vi.fn(), disconnect: vi.fn() }
        }),
      },
    },
  } as unknown as HTMLDivElement
  const options = () => {
    const snapshot = keys
    return {
      count: snapshot.length,
      estimateSize: () => 100,
      getItemKey: (index: number) => snapshot[index]!,
      getScrollElement: () => scrollElement,
      scrollToFn,
      observeElementRect: (_: any, cb: (rect: { width: number; height: number }) => void) => {
        cb({ width: 400, height: 300 })
        return () => {}
      },
      observeElementOffset: (_: any, cb: (offset: number, isScrolling: boolean) => void) => {
        cb(scrollElement.scrollTop, false)
        return () => {}
      },
      anchorTo: 'end' as const,
      paddingStart: padding,
    }
  }
  const virtualizer = new Virtualizer(options())
  virtualizer._willUpdate()
  virtualizer['getMeasurements']()
  scrollToFn.mockClear()
  const update = () => {
    virtualizer.setOptions(options())
    ;(scrollElement as any).scrollHeight = padding + keys.length * 100
    virtualizer._willUpdate()
  }
  return {
    scrollToFn,
    prepend(count: number) {
      keys = [...Array.from({ length: count }, (_, i) => `p${i}`), ...keys]
      update()
    },
    setPaddingStart(next: number) {
      padding = next
      update()
    },
  }
}

test('control: a prepend keeps the item at the offset (m2 starts at 240, offset 250)', () => {
  const { prepend, scrollToFn } = create(40, 250)
  prepend(8)
  expect(scrollToFn).toHaveBeenCalledTimes(1)
  expect(scrollToFn.mock.calls[0]![0]).toBe(1050)
})

test('a paddingStart-only change keeps the item at the offset', () => {
  const { setPaddingStart, scrollToFn } = create(40, 250)
  setPaddingStart(80)
  // m2 moved from 240 to 280; keeping it in place needs offset 290.
  expect(scrollToFn).toHaveBeenCalledTimes(1)
  expect(scrollToFn.mock.calls[0]![0]).toBe(290)
})

test('a reader inside the padding keeps the first item when the padding grows', () => {
  const { setPaddingStart, scrollToFn } = create(40, 33)
  setPaddingStart(80)
  // m0 was at viewport y 7 (40 - 33); it stays there at offset 73.
  expect(scrollToFn).toHaveBeenCalledTimes(1)
  expect(scrollToFn.mock.calls[0]![0]).toBe(73)
})