Has this been tested on Mac, Ubuntu and Windows ? (not just CI but actually testing that the tutorial windows appear correctly etc.)

I have tested tutorials 102, 107 and 108 on my Archlinux and they work as expected. I can test on Windows too since we don't have many Windows users, but I don't have a Mac.

Also tested on Windows, and it works fine. I also fixed a cmake issue with top-level project detection (somehow was not working on Windows).

@jdumas @alecjacobson just wanted to let you know: everything looks fine and behaves as I'd expected it on my MacBook(macOS Mojave 10.14.4)!
I've tested 102, 107 and 108 as well as 106 & 708.

To do so I took the following steps:

git clone https://github.com/libigl/libigl.git
cd libigl
git checkout dev
git pull origin update-libs
mkdir build
cd build
cmake ../
make -j4