Thanks for sharing, your code looks correct. I confirm setting the default renderer won’t help as vtkResliceCursorPicker consider the intersection to be valid even when the provided point is outside the renderer.
I am not sure there is a way to fix this without modifying VTK. The right approach would be to early return in vtkResliceCursorPicker when the point is outside the viewport, or at least modify vtkResliceCursorPicker::IntersectPolyDataWithLine to return 0 when vtkCell::IntersectWithLine returns a parametric coord outside the [0 - tol, 1 + tol] range.
Maybe you can find a way to add observers on mouse events, and do the viewport check in your callback function to disable interactions on widgets outside the viewport.
Feel free to contact me directly if you would like Kitware to work on the fix in the context of a support contract.