There are times in one’s life when one becomes so fed up with Windows 10 and its shenanigans that one decides to install Ubuntu instead. At such a time, one may wish to keep the previous installation of Windows, should for some reason one want to use Windows 10 again.
It is therefore desirable to shut down Windows normally, and not merely hibernate it, before proceeding to resize the Windows partition. Lest, of course, one corrupt the Windows partition on waking it so badly that it will no longer boot subsequently, and without even a restore CD to repair it.
Oh well. I was really fed up with Windows 10 anyway. (To wit: there seems to be a vast misunderstanding over who actually owns the machine.)
The good news: I write on an entirely separate computer, and have decent backups. I have met this discovery with stoicism and amusement. I was really only using Windows for gaming, and with (most!?) Steam games running on Ubuntu it hardly matters.