I found it, thanks.
On that note: I think the phenomenon I described in the original post is not a Budgie or DE functionality at all, but one of MPV! I think I automatically associated the functionality with the DE because that was what I was expecting, coming from KDE Plasma.
I have today replicated the behavior for a short time, and I could see that the dragging seemed to scale and/or alter the video inside the MPV, not the while window itself. Therefor I would think that it is an MPV functionality.
Thanks for your help.