This is one of the reasons I've given up on using Tk for Sigil plugin GUI interfaces. Too many platform-specific quirks (not to mention that macOS is the only one smart enough to automatically match Tk to the light/dark os theme). Pop-unders have been a problem on all platforms at different times.
I never did warm up to Tk. I was never able to get a good feel for it (programmatically speaking). It just never felt intuitive to me.
But I'm glad you found a workaround for the issue! I wouldn't dream of suggesting that plugin devs overhaul their plugins to use PySide instead. "If it ain't broke," and all that.
