Errors with Customized ScrollBar Sizing Porting Qt 5.15 QML to Qt 6.0.2 QML

Question

I am transitioning a GUI from Qt 5.15 QML to Qt 6.0.2 QML but an error has occurred in ScrollBar behavior. I have discovered the issue with MacOS 10.15.7 and I don't know where it exists elsewhere. I create a minimal custom ScrollBar and attach it to a ListView. The scroll indicator works fine in 5.15 but in 6.x the scroll indicator fills the entire scroll regardless of whether or not it should. Has behavior changed or is this a bug?

I create CustomScrollbar.qml as follows:

import QtQuick 2.15
import QtQuick.Controls 2.15
ScrollBar {
    contentItem: Rectangle {
        implicitWidth: 15
        color: "red"
    }
}

and then use it in a basic ListView:

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.Controls 2.15
Window {
    width: 300; height: 100; visible: true

    ListView {
        id:listView
        anchors.fill:parent
        ScrollBar.vertical: CustomScrollbar {}

        model: ListModel {
            ListElement {name: "Bill"} ListElement {name: "Bob"} ListElement {name: "Andy"} ListElement {name: "Jim"}
            ListElement {name: "Ralph"} ListElement {name: "Ed"} ListElement {name: "Bill"} ListElement {name: "Bill"}
        }
       delegate: Text {
           text: name
       }
    }
}

Qt 5.15 produces a properly positioned indicator (left) but Qt 6.0.2 does not scale properly and does not scroll:

Setting the matter aside, I'd suggest using properties [`horizontal`](https://doc.qt.io/qt-5/qml-qtquick-controls2-scrollbar.html#horizontal-prop) and `vertical` in you custom scroll bar component to bind its `implicitWidth` and height conditionally. — ratijas, Mar 17 '21 at 08:00

score 0 · Answer 1 · answered Apr 01 '21 at 18:56

At the time of this writing the issue appears to be accepted as a verified bug in Qt 6 (see here) that has been assigned a P2 - Important level. As of several months before this post, an article by Phoronix notes that there are over 1000 P1 - Critical bugs languishing in Qt so it is unclear if the lower level P2 will see attention soon.

It might be possible to use an external, non-attached scrollbar to workaround this issue but care would need to be taken to not corrupt flickable behavior in the process.

Errors with Customized ScrollBar Sizing Porting Qt 5.15 QML to Qt 6.0.2 QML

1 Answers1