Summary

ScrollBar can't be anchored to header/footer of a ListView even if they are positioned as ListView.Overlay*.

Description

The thing that I wanted probably can be built using ColumnLayout, but for the hell of it I tried a ListView's natively supported propertied: header and footer. For simplicity's sake, let's leave out the footer for now. So, we have a ListView, a sticky header at the top, and a flickable content. Header always stays at the top, so naturally I tried to restrict ScrollBar's bounds to the actually scrollable area.

        ScrollBar.vertical: ScrollBar {
            parent: listView
            anchors.top: listView.top
            anchors.bottom: listView.bottom
            anchors.right: listView.right
            anchors.topMargin: listView.headerItem ? listView.headerItem.implicitHeight : 0
            anchors.bottomMargin: listView.footerItem ? listView.footerItem.implicitHeight : 0

            policy: ScrollBar.AlwaysOn
        }

It kinda succeeded for header (except ListView.OverlayHeader is broken, pretty much failed for footer (scrollbar is at full height). And left multiple identical warnings in console for both.

Note that anchors are bound as margins to implicit sizes, not as top/bottom of the items themselves.

Steps to reproduce

Run the code in the attached example.

Excepted output

The code looks like it should work as it is. Especially given that header is an OverlayHeader, so it's size and position shouldn't change at all.

Actual output

qrc:/main.qml:35:29: QML ScrollBar: Possible anchor loop detected on vertical anchor.
qrc:/main.qml:35:29: QML ScrollBar: Possible anchor loop detected on vertical anchor.
qrc:/main.qml:35:29: QML ScrollBar: Possible anchor loop detected on vertical anchor.