Manifold::Initialize() sqrt of square not required?

sf = std::sqrt( A->staticFriction * A->staticFriction );

Is there a reason this is done like so? Something unfinished, perhaps?

PS thank you so much for a clear 2D working example. I looked high and low for something similar to help me debug my own version. In the end I implemented a version of ImpulseEngine in Processing 4.

What's wrong with it?

Ah yeah it should mix A and B together, it's a typo :)