[PMA] Dragging a window outside the title bar causes the mouse position to shift when changing DPIs

Copied from VSO ID: 780675

Min Repro Proj:
[WpfWeirdDrag.zip](https://github.com/dotnet/wpf/files/3291838/WpfWeirdDrag.zip)

This bug is found during PMA testing. To setup PMA review the VSO ID

Repro:

Open Update Dialog in main window.
Left click on the spinning circle, and drag to a secondary window.

Expected: After dragging the UI to the secondary Window, the mouse pointer is in the same location.

Actual: After dragging the UI to the secondary Window, the mouse pointer is repositioned to a different location.

Note: The dialog's title bar doesn't have this issue. Other VS UI appears to have similar issue.