Steps to reproduce:

1. Execute this code:

import QtQuick 2.4

Item {
    id: root

    width: 560
    height: 640

    Rectangle {
        color: 'tomato'
        width: parent.width / 10
        height: parent.height / 10

        MouseArea {
            anchors.fill: parent
            drag.target: parent
            acceptedButtons: Qt.LeftButton
        }
    }
}

2. Drag the item
3. Right click while dragging

What happens:

After clicking with the right button of the mouse, the item jumps to other coordinates as soon as we start to move the mouse again. If acceptedButtons is set to Qt.LeftButton | Qt.RightButton instead, it looks like everytime a click happens, the value of drag.threshold need to be reached again before the item can move.

What should happen:

The item should ignore any other click while being dragged by the user.