Maximum-average-degree conjecture for non-2-mixing oriented graphs

Let an oriented graph be a digraph with no pair of oppositely directed parallel arcs, and let its maximum average degree be the maximum of $2|A(H)|/|V(H)|$ over all non-empty subdigraphs $H$. A digraph is 2-mixing if any two of its 2-dicolourings can be connected by single-vertex recolourings through 2-dicolourings. Oriented-graph density conjecture. Every non-2-mixing oriented graph has maximum average degree at least $4$. This is supported in the paper by proving the claim for 2-freezable oriented graphs; the general conjecture remains open.