Stratifying quiver Schur algebras via ersatz parity sheaves

We propose an extension of the theory of parity sheaves, which allows for non-locally constant sheaves along strata. Our definition is tailored for proving the existence of (proper, quasihereditary, etc) stratifications of $\mathrm{E}\mathrm{x}\mathrm{t}$-algebras. We use this to study quiver Schur algebras $A(\alpha )$ for the cyclic quiver of length $2$. We find a polynomial quasihereditary structure on $A(\alpha )$ compatible with the categorified PBW basis of McNamara and Kleshchev-Muth, and sharpen their results to arbitrary characteristic. We also prove that semicuspidal algebras of $A(n\delta )$ are polynomial quasihereditary covers of semicuspidal algebras of the corresponding KLR algebra $R(n\delta )$, and compute them diagrammatically.