Some sort of "bleed" between these objects

Unity Engine
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1

This is a video player but the UI parts are a cube (black) and a panel (white). There is space between the box and the panel. The white material is Opaque and it is all URP. The box is leaking thru because if I make the box red the interference is red. Have I made some fundamental error?

2

I worked around it by changing the video player to use Render Mode = Render Texture and using a cube for the video panel.