Lo and behold… apparently, Sets is implemented by drawing over the titlebar that was already drawn, rather than modifying the actual titlebar itself. Which would work in theory, but in a VM this happens all the time…
Considering that the titlebar is drawn by the OS (not the UI, in most cases), Microsoft could just have modified the titlebar itself unless changes to the titlebar were detected (e.g. ribbons in titlebar). However, this approach really, really shows when resizing a window.
I don’t know if it is noticeable on most real computers, but in my VM setup it is honestly like half the time I resize a window. Also, note that this stays after dragging – all the screenshots were taken after I let go of resizing the window.