I believe the issue here is that the code to decide if a pixel is on the boundary of the label region is taking into account the 3D neighbors when it should only consider the 2D neighbors (because in lightbox mode the layers are extracted as a 3D volume and then split up per-cell. We should probably move the boundary detection code further down the pipeline (if possible) or else add a mode in the vtk filter so that it treats the slices independently. Perhaps modifying the vtk filter is the more straightforward and more generally useful solution.

This isn't a scene views issue so much as a light box issue, Steve who do you suggest should be assigned to it?

I'll assign this to myself for now - I think it's an issue with the vtk filter used to create the outline volumes not working correctly in slice mode. It'll take a bit of work to make it behave as expected in this mode due to the way lightboxes are implemented.

The code is here:

https://github.com/Slicer/Slicer/blob/master/Libs/MRML/Logic/vtkImageLabelOutline.cxx

It may be as simple as changing this line to SetNeighborTo4 instead of 8:

https://github.com/Slicer/Slicer/blob/master/Libs/MRML/Logic/vtkImageLabelOutline.cxx#L33

But I haven't tested it.

I just tested it, but changing 8 to 4 doesn't resolve the problem.

Thanks for checking - after a little closer look, it probably comes down to (optionally) replacing the tripple nested for-loop construct with a double nested one that only looks in-plane.

https://github.com/Slicer/Slicer/blob/master/Libs/MRML/Logic/vtkImageLabelOutline.cxx#L135