VTK pane renders outside Panel

Hi all, VTK seems to work great in Panel, but I’m getting a window that’s somehow being rendered outside of Panel, and can’t figure out why. There’s no interactor on it, nothing fancy. It just paints a sphere. The render works, it looks great in a Panel pane. But just before that there’s a little pop-up that appears at the top left corner of my screen, that contains the same sphere. Somehow it’s trying to render to an OpenGL window or something. The pop-up goes away the instant the Panel pane appears.

My code is:

ren = vtk.vtkRenderer()
renWin = vtk.vtkRenderWindow()
renWin.AddRenderer(ren)

actor = create_sphere()
ren.AddActor(actor)

ren.SetBackground(.1,.1,.1)
renWin.Render()

mypane = pn.pane.VTK(renWin)

The same thing happens on a re-render, a little pop-up appears just before the Panel pane.