Real variations of stability on K3 categories

This paper proves that the 2-Calabi-Yau triangulated category associated with the preprojective algebra of an affine or hyperbolic graph admits many real variations of stability conditions, in the sense of Anno, Bezrukavnikov, and Mirkovi\'c. We do this by connecting the Coxeter arrangements of the graph with the homological algebra by introducing the concept of real flows. This categorifies the notion of above and below for alcoves in a hyperplane arrangement and allows us to generalise much of the machinery of real variations for affine arrangements to the hyperbolic setting. In the process, we show that the derived category with nilpotent cohomology is equivalent to the derived category of nilpotent modules over the preprojective algebra.