With Attached example where a red border rectangle is created to display the MouseArea in the qml. Drag event is added so that dragging can be done horizontally (Drag.XAxis).
On dragging the mouse over the red rectangle, we can see the value of MouseX and X . MouseX position depends on Width of the parent with Qt 5.15.0 which is not the case in Qt 5.12.4. It shows (-(x position wrt parent rectangle)) in Qt 5.15.0.