HarmonyOS scrollbar显示不出来

参考代码：

// xxx.ets
@Entry
@Component
struct ScrollBarExample {
  private scroller: Scroller = new Scroller()
  private arr: number[] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
  private isScrollTouch: boolean = false

  build() {
    Column() {
      Stack({ alignContent: Alignment.End }) {
        Scroll(this.scroller) {
          Flex({ direction: FlexDirection.Column }) {
            ForEach(this.arr, (item: number) => {
              Row() {
                Text(item.toString())
                  .width('80%')
                  .height(60)
                  .backgroundColor('#3366CC')
                  .borderRadius(15)
                  .fontSize(16)
                  .textAlign(TextAlign.Center)
                  .margin({ top: 5 })
              }
            }, (item: number) => item.toString())
          }.margin({ right: 15 })
        }
        .width('90%')
        .scrollBar(BarState.Off)
        .scrollable(ScrollDirection.Vertical)
        .onScrollFrameBegin((offset: number, state: ScrollState) => {
          if (this.isScrollTouch) {
            return { offsetRemain: 0 }
          }
          return { offsetRemain: offset }
        })
        .onTouch(() => {
          if (!this.isScrollTouch) {
            this.isScrollTouch = true
          }
        })

        ScrollBar({ scroller: this.scroller, direction: ScrollBarDirection.Vertical, state: BarState.Auto }) {
          Text()
            .width(20)
            .height(100)
            .borderRadius(10)
            .backgroundColor('#C0C0C0')
        }.width(20).backgroundColor('#ededed')
        .onTouch(() => {
          if (this.isScrollTouch) {
            this.isScrollTouch = false
          }
        })
      }
    }
  }
}