Offset of start of anchor item in pixels from the top of the visible content. May be negative
to indicate that the anchor item starts before the visible viewport.
anchorIndex
anchorIndex:number = 0
Index of list element that serves as an anchor for positioning the rendered elements relative
to the scroll container.
Offset of start of anchor item in pixels from the top of the visible content. May be negative to indicate that the anchor item starts before the visible viewport.