After patching my 3.7.2 Macbook installation, and reviewing #20406 which added the icon code, I understand this issue better.  IDLE, while open and unlike other Mac apps I later tested, was replacing the initial Mac IDLE app dock icon (tilted page with double snakes and pen) with the cross-platform IDLE icon.

This was an accidental side-effect.  #20406 replaced the tk icon on the title bar of IDLE windows with the little IDLE icons.  Before merging, I checked that this did not affect the large app icons on the taskbar.  I assume that Serhiy did the equivalent on Linux.  I don't think that the patch was visually checked on Mac.

After the patch, I noticed that the normal behavior with macOS is to put a barely visible (to my eyes) black dot under dock icons when an app is open.  On Windows, the icon is also left as is, but a much more visible icon-width underline is added while open.  I approve of removing the while-open overlay on Mac and don't think it needs to be reinstated.
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Kevin, I suspect that higher resolution .pngs (64px, 128px?) would help on other *nixes, (and perhaps Windows), but this would be another issue.  Whatever is in the .ico file for Windows looks awful in the corner of IDLE windows on my system, but this also is another issue.