I am not Windows developer and sure I didn't know this "Windows-specific construct". But I can google https://www.google.com/search?q=Multiple+Top-Level+Windows

I am not Windows developer and sure I didn't know this "Windows-specific construct". But I can google https://www.google.com/search?q=Multiple+Top-Level+Windows+Interface https://www.google.com/search?q=Multiple+Top-Level+Windows+I... Also I can look on the link I post and Ctrl+F "Multiple Top-Level Windows Interface" > Microsoft Word 2003: MDI until Microsoft Office 97. After 2000, Word has a Multiple Top-Level Windows Interface, thus exposing to shell individual SDI instances, while the operating system recognizes it as a single instance of an MDI application.