Attribute of a UserControl does not update the y

I've got a UserControl (winforms) that is placed onto a Form (actually a Weifen.Luo.DockContent , that again derives from the basic form). AutoScroll is enabled for the form and the control can be moved on it by dragging around (more precisely by evaluating MouseDown , MouseMove , MouseUp and updating the Location ).

Sometimes* when reaching the border (so that the form's plane must be further enlarged) updating the location of the control suddenly stops working; setting Location = .. fails. For demonstration a snippet from the immediate window while debugging:

>Location
{X = 50 Y = 751}
    IsEmpty: false
    X: 50
    x: 50
    Y: 751
    y: 751

But after updating...

>Location = new Point(30, 40);
>Location
{X = 30 Y = 751}
    IsEmpty: false
    X: 30
    x: 30
    Y: 751
    y: 751

.. the y-Value still remains the same. How can this be? It seems as there is an LocationChanged-Handler attached (outside user code) thats works against me. Any hints?

*) allthough I wasn't able to find the exact cases when it happens, I found ways to reproduce it, eg when focusing a textbox on the control and then moving it to the edge or adding an (empty!) Load event handler to the control - weird. I know this sounds more like a race condition. But currently I've got no idea where to go on searching.

---

Update: Every time the update fails, the Parent-Form's AutoScrollPosition gets updated instead, resulting so that delta(AutoScrollPosition before and after) = delta(Location, newValueForLocation).

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