This is both a correction, and an improvement to the creation of a quaternion from two vectors. Or as it is commonly known, Shortest RotationArc.
The current code doesn’t handle the case where the two vectors are opposing; there’s an if statement to detect it, and a comment, but no recovery code. The problem is that there are an infinite number of orthogonal rotation axes — we just need to pick one. The solution is to check to see which axis the start vector (it could be the finish vector, it doesn’t matter) is generally pointing along. If it’s pointing generally along the -axis, then we take the cross product with the -axis to get our rotation axis. Otherwise we take the cross product with the -axis.