I've played around with using the 3DCamera.CanvasToLayer expression to attempt to place objects on the flat x/y plane.
Set sprite.x to 3DCamera.CanvasToLayerX("Layer 0", Mouse.x, Mouse.y, 0)
Set sprite.y to 3DCamera.CanvasToLayerY("Layer 0", Mouse.x, Mouse.y, 0)
this seems to work correctly, ONLY when "full screen mode" is set to OFF
Any other setting will cause the sprite to be offset to some degree, and it changes if you rescale the viewport etc.
I've tried playing around with setting the project to Normalized vs Regular z axis scaling, but no luck.
Is there any way to make this work with different window resolutions?