Relative position triggers drag under wrong circumstances

[VacuumBreather]

Describe the bug
The library uses a relative mouse position to determine whether a drag operation has started. Unfortunately this position is relative to a UI Element, not the MainWindow. This has the result that if a "ScrollIntoView" operation is triggered by a click, the drag operation is started, despite the user not having moved the mouse. Instead the element that was clicked on has moved, therefore changig the relative position.

To Reproduce
Steps to reproduce the behavior:

1. Create a ListBox with large enough item container styles.
2. Scroll so one element is only half in-view.
3. Click on the element and hold the mouse button down.
4. Observe that the element gets scrolled fully into view, which triggers the drag operation

Expected behavior
Drag should only happen when the mouse is moved, not when an element is moved.