Edit: Pavel has shown that it may not be VisualBrush's fault so I have renamed the question for my specific problem.
The example is a WPF Datagrid; Scroll right until a column header is partially visible. drag the partially visible column header to reorder it. the drag indicator is the partial view of the header (not complete)
My Solution - Still interested in other solutions
I subscribe to ColumnReordering and replace the drag indicator with my own.
Copying the source of DataGridColumnFloatingHeader for my own class.
I change the line that creates the visual brush using the header, to a line that creates the visual brush using the first child of the header (which is not clipped)
remove the offset and use 0,0..