Wrong Flickable can get mouse grab when there are 3 or more nested interactive items.

If multiple nested Flickables with non zero pressDelays filter another nested items mouse events then the outer Flickable can incorrectly receive mouse grab when the inner Flickable should get it.

In the example below if you click (press and release) on one of the list items and then move the mouse to the side some distance and attempt to drag the list down it will not move down, instead while the mouse button is held down horizontal movements will drag the outer list view side to side.

The basic reason is mouse event filtering is done from innermost item out and the inner most Flickable with a pressDelay will accept mouse press events preventing the outer items from filtering them. This means they compare the positions of move events with a previous (replayed) press event and if that's far from the start of the drag the movement appears to have exceeded the drag distance and the mouse is grabbed.

import QtQuick 2.0

Rectangle {

    width: 480
    height: 640

    ListView {
        width: 480
        height: 640
        spacing: 5
        orientation: ListView.Horizontal
        pressDelay: 200
        model: [
            "red",
            "green",
            "blue"
        ]
        delegate: ListView {
            property color rowColor: modelData
            width:  320
            height: 640
            spacing: 5
            model: 30
            pressDelay: 200
            delegate: Rectangle {
                width: 320
                height: 30
                radius: 3
                color: rowColor
                
                MouseArea {
                    anchors.fill: parent
                }
            }
        }
    }
}