Example:

QML:

import QtQuick 2.12
import QtQuick.Window 2.12

Window {
    width: 500
    height: 500
    visible: true

    Rectangle {
        x: 0
        y: 0
        width: 5
        height: 20
        visible: true

        MouseArea {
            drag.target: parent
            drag.axis: Drag.XAxis
            drag.minimumX: 0
            drag.maximumX: 250
            drag.threshold: 0
            drag.smoothed: false

            onMouseXChanged: {
                console.log("Rect X: " + parent.x);
                console.log("Mouse X (relative to Rectangle): " + mouseX);
                console.log("Mouse X (relative to Window): " + (parent.x + mouseX));
            }
        }
    }
}

The following output appears when moving the mouse from 0 to 250:

QDEBUG : Drag::Test_SlideFromLeftToRight() qml: Mouse X (relative to Rectangle): 4
QDEBUG : Drag::Test_SlideFromLeftToRight() qml: Mouse X (relative to Window): 247
QDEBUG : Drag::Test_SlideFromLeftToRight() qml: Rect X: 247
QDEBUG : Drag::Test_SlideFromLeftToRight() qml: Mouse X (relative to Rectangle): 4
QDEBUG : Drag::Test_SlideFromLeftToRight() qml: Mouse X (relative to Window): 251
QDEBUG : Drag::Test_SlideFromLeftToRight() qml: Rect X: 250
QDEBUG : Drag::Test_SlideFromLeftToRight() qml: Mouse X (relative to Rectangle): 3
QDEBUG : Drag::Test_SlideFromLeftToRight() qml: Mouse X (relative to Window): 253

Description

In short: The relative mousecoordinate is wrong when you drag an item (in this example an Rectangle) via an embedded MouseArea. Mouse X (relative to Window) should never be greater 250, but in this example it is (251, 253)! The mouse has always an offset.

Condition: MouseArea global position must depend on position of the dragged item (in this example MouseArea is parent of Rectangle).

The Problem: The rectangle is moved to the new coordinates but the relative mousecoordinates are NOT "corrected".

Analysis: qquickmousearea.cpp

void QQuickMouseArea::mouseMoveEvent(QMouseEvent *event)

If dragged item has impact on global position of MouseArea the mousecoordinate should be "corrected"

A demo is attached.

Workarround

For users with the same issue:

        MouseArea {
            property var oldX : 0;

            drag.target: parent
            drag.axis: Drag.XAxis
            drag.minimumX: 0
            drag.maximumX: 250
            drag.threshold: 0
            drag.smoothed: false

            onMouseXChanged: {
                console.log("Rect X: " + parent.x);
                console.log("Mouse X (relative to Rectangle): " + mouseX);
                console.log("Mouse X (relative to Window): " + (parent.x + mouseX));
                console.log("Mouse X (relative to Window correct): " + (parent.x + mouseX - (parent.x - oldX)));

                oldX = parent.x
            }
        }