Sobolev stability of the $L^2$-projection on hybrid meshes

We establish $L^p$- and $W^{1,p}$-stability of the $L^2$-projection onto mapped Lagrange finite elements on hybrid meshes consisting of triangles and convex quadrilaterals arising from adaptive mesh refinement. If $K$ is the (tensor product) degree of polynomials of the discretisation, then we show, in particular, $W^{1,2}$-stability for all $K\ge 2$ for the Q-RG and Q-RB refinements. This extends results by Ali, Funken, and Schmidt (2022) which hold for the range $2\le K\le 9$ for initial meshes consisting of parallelograms. Our proof relies on an extension of the technique by Diening, Storn and Tscherpel (2021) to general convex quadrilaterals.