Changing the Slab size ends up working like a zoom for large slabs.

[JamesAPetts]

An alternative approach is now that we are using parallel projection, we can keep the camera position outside of the volume, and just move the near and far clipping planes along with the focal point, which will be the center of the target slice. Zoom is then achieved by making the parallelScaling of the frustum larger and smaller.

This will also come with the benefits of not having to worry if the camera is within the MIP volume.

[JamesAPetts]

This was actually fixed by the parallel fragment shader fix: Kitware/vtk-js#1234