The trunk and 8.6.7 branch of Tk on macOS have recently implemented the wm_iconphoto command, which had not previously been supported on macOS. This means that versions of IDLE that link to this version of Tk will inherit the iconphoto behavior on Windows and X11, which results in a extremely blurry icon image in the Dock. It would probably be best to make this command conditional on macOS to just retain the standard app icon, or else add a sharper image.