For now, I’m going to disable VTK on the GitHub nightly builds.
While we now have support to compile against VTK9, nothing actually works because of changes between the VTK 8 and VTK 9 interfaces.
Consequently, we’re working on some contingency plans, particularly for charts / plots.
In the meantime, I’m going to turn off VTK builds so anyone testing Mac and Windows (and AppImage) gets a better experience.