QML ListView nesting GridView, the ScrollBar behavior is not natural

[minquan.li]

I have a requirement about show the Photo gallery group by datetime.
I use a QML ListView nesting GridView,It looks like:

It looks well unit I add a ScrollBar in ListView:

Scrollbar.vertical : Scrollbar {
    hoverEnabled : true
    active : hovered ||pressed
    orientation : Qt . Vertical
    anchors.top : parent . top
    anchors.right : parent . right
    anchors.bottom : parent , bottom
    contentItem : Rectangle {
        timplicitwidth : 6
        implicitheight : 100
        radius : width / 2
        color : "#0f1c3e"
    }
}

When I scroll in ListView The scorllbar changed it's size and position on changed the currentItem in ListView.

I don't know why about it,but I found the contentHeight will change to the currentItem height after scroll the listView.
I want get the actual behavior scroll bar,What need I do?