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Details

Type: Bug
Resolution: Done
Priority: P1: Critical
Fix Version/s: 6.3.1, 6.4.0 Beta1
Affects Version/s: 5.15.1
Component/s: Quick: Mouse Touch and Tablet input
Labels:
None

Commits:
d92f1dfe05 (qt/qtdeclarative/dev) d92f1dfe05 (qt/tqtc-qtdeclarative/dev) 07e40cf567 (qt/qtdeclarative/6.3) 07e40cf567 (qt/tqtc-qtdeclarative/6.3) 1fe90c8d2e (qt/tqtc-qtdeclarative/5.15) 07e40cf567 (qt/tqtc-qtdeclarative/6.3.1)

Description

first_ok.webm second_ng.webm The PreventStealing of mouseArea does not work properly in Flickable's(ListView) child item.

If you look at the test application, the configuration is as follows.

ListView
Custom ScrollBar
The test application is using ScrollBar for ListView. The ScrollBar has been customized and added handles for quick scrolling.
We set the preventStealing property to true on the handle of the ScrollBar.

The screenshot of running the application is as follows.

Test Application codes

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.Controls 2.12

Window {
    id: root
    width: 640
    height: 480
    visible: true
    title: qsTr("testApplication")

    property int dynamicModel: 100

    Rectangle {
        anchors.fill: parent
        Component {
            id: contactDelegate
            Rectangle {

                border.color: "#81e889"
                width: 640; height: 100
                Column {
                    anchors.centerIn: parent
                    Text { text: '<b>Name:</b> ' + index }
                    Text { text: '<b>Number:</b> ' + index }
                }
            }
        }

        ListView {
            id: list1
            model: root.dynamicModel
            delegate: contactDelegate
            highlight: Rectangle { color: "lightsteelblue"; radius: 5 }
            focus: true
            width: 640
            height: 480
            ScrollBar.vertical: ScrollBar {
                id: control
                size: 0.3
                position: 0.2
                active: true
                orientation: Qt.Vertical

                contentItem: Rectangle {
                    id: handleBar
                    implicitWidth: 6
                    implicitHeight: 100
                    radius: width / 2
                    color: control.pressed ? "#81e889" : "#c2f4c6"

                    Rectangle {
                        id: handle
                        height: 64
                        width: 64
                        anchors.right: handleBar.left
                        anchors.top: handleBar.top
                        visible: true
                        radius: height / 2
                        color: handleControl.pressed ? "#17a81a" : "#21be2b"

                        Text {
                            anchors.centerIn: parent
                            text: "Drag me"
                        }

                        MouseArea {
                            id: handleControl
                            anchors.fill: parent

//////////////////////////////////////////////////////////////////
// set preventStealing to true
//////////////////////////////////////////////////////////////////

                            preventStealing: true
                            drag {
                                target: control.contentItem
                                minimumY: 0
                                maximumY: control.height - handle.parent.height
                                axis: Drag.YAxis
                            }

                            onPositionChanged: {
                                if (pressed) {
                                    control.parent.contentY = control.contentItem.y
                                            / control.height * control.parent.contentHeight
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

Steps to reproduce the issue

1. Run test application
2. Flicking ListView
3. Click and Drag handle when ListView is moving (It's OK!) <---- first_ok.webm
4. Flicking ListView again after ListView is stopping
5. Do step 3 again. (It's a bug) <---- second_ng.webm

Clicking and dragging the handle does not work. In this situation, the ListView is receiving click and dragging events.
Expected results : We expect the handle to be clicked and the scrollbar to move through dragging.
After the movement of the ListView is completely stopped, you can click or drag the handle. But We want it to work(click and drag) when it moves.

The Issue analysis

The reason why it doesn't work properly on the second try
The event is stolen because receiverKeepsGrab is returned as false in the filterMouseEvent function. keepMouseGrab is returned as false. See below the link.
https://code.woboq.org/qt6/qtdeclarative/src/quick/items/qquickflickable.cpp.html#2582
Why is keepMouseGrab set to false?
keepMouseGrab is set to false in the mouseReleaseEvent function.
https://code.woboq.org/qt5/qtdeclarative/src/quick/items/qquickmousearea.cpp.html#823
Why it works normally on the first try
In the first attempt, the mouseReleaseEvent function has not yet been executed. So, keepMouseGrab is set by setPreventStealing
https://code.woboq.org/qt5/qtdeclarative/src/quick/items/qquickmousearea.cpp.html#572

Our approach is as follows

Fixed KeepMouseGrab not to be set to false when preventStealing is set.

void QQuickMouseArea::mouseReleaseEvent(QMouseEvent *event)
{
    Q_D(QQuickMouseArea);
    d->stealMouse = false;
    d->overThreshold = false;
    if (!d->enabled && !d->pressed) {
        QQuickItem::mouseReleaseEvent(event);
    } else {
        d->saveEvent(event);
        setPressed(event->button(), false, event->source());
        if (!d->pressed) {
            // no other buttons are pressed
#if QT_CONFIG(quick_draganddrop)
            if (d->drag)
                d->drag->setActive(false);
#endif
            // If we don't accept hover, we need to reset containsMouse.
            if (!acceptHoverEvents())
                setHovered(false);
            QQuickWindow *w = window();
            if (w && w->mouseGrabberItem() == this)
                ungrabMouse();

// this codes !!!!
            if (!preventStealing())
                setKeepMouseGrab(false);
        }
    }
    d->doubleClick = false;
}