HarmonyOS 鸿蒙Next 关于 Scroll 组件疑问

import { hilog } from '@kit.PerformanceAnalysisKit'
import { LengthMetrics } from '@ohos.arkui.node'

@Entry
@ComponentV2
struct ChatIndex {
  @Local list: number[] = []
  listScroller: ListScroller = new ListScroller()
  firstIndex: number = 0

  aboutToAppear(): void {
    for (let i = 0; i < 10; i++) {
      this.list.push(i)
    }
  }

  build() {
    Column() {
      Text("对话")
      Divider().width('100%').backgroundColor(Color.Gray)
      List({ scroller: this.listScroller, initialIndex: this.list.length - 1 }) {
        Repeat<number>(this.list)
          .each((ri: RepeatItem<number>) => {
            Text(ri.item.toString())
              .height('20%')
          })
          .key((item: number, index: number) => item.toString())
          .virtualScroll()
      }
      .divider({
        color: Color.Blue,
        strokeWidth: '5vp'
      })
      .layoutWeight(1)
      .onScrollVisibleContentChange((start) => {
        hilog.debug(0x000000, 'rainrain', 'start == ' + start.index)
        this.firstIndex = start.index
      })

      Divider().width('100%').backgroundColor(Color.Gray)
      Row() {
        Button('在头部新增，但不影响影响List 展示')
          .onClick(() => {
            hilog.debug(0x000000, 'rainrain', 'start == ' + this.listScroller.getItemRect(this.firstIndex).y)
            let offset: number = this.listScroller.getItemRect(this.firstIndex).y
            let top = 0
            if (this.list.length != 0) {
              top = this.list[0]
            }
            let addCount = 4
            for (let index = 0; index < addCount; index++) {
              top--
              this.list.unshift(top)
            }
            this.listScroller.scrollToIndex(this.firstIndex + addCount, false, ScrollAlign.START, {
              extraOffset: LengthMetrics.vp(-offset)
            })
          })
        Blank()
        Button('新增消息').onClick(() => {
          this.list.push(this.list.length)
          if (this.listScroller.isAtEnd()) {
            this.listScroller.scrollToIndex(this.list.length - 1, true, ScrollAlign.END)
          }

        })
      }
    }
  }
}