[damonbrodie]

To stress it again, before the 1709 Fall Creators Update, Kodi was doing just fine, which means - to me - that it cannot be related to my Kodi settings. I really hope there's some solution, other than to change to non-windowed full screen.

 The problem seems to not be Kodi, but some combination of Windows and/or video drivers since the update.  As far as I have read, the only cures involve:
-Applying all the latest Windows patches
-Updating to the latest video card patches
-Switching to Full Screen Windowed
-Disabling the refresh rate changing on content

There may yet be fixes coming from Microsoft and/or via Video Drivers, however I think the Kodi team (based on PatK's response) consider this not to be a Kodi bug.