I will try to recreate it on my dev box (High Sierra). This is probably a Qt issue so .... we may not be able to get an immediate fix so ...
Exactly how are you closing that Window? Are you hitting the red button on the titlebar, or actually using AppleKey+Q or using the File->Close/Quit button?
Does how you close impact the issue? Does this happen if the epub itself has been modified causing the "are you sure" message?
KevinH
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