Hi - the bug in the video is the one above, mentioned by @AnTi. It's fixed ready for the next release, but I had to take a really heavy-handed approach due to some nasty shortcomings in one of libraries MyPaint uses, called "GTK".
I had to disable snapping out subwindows on the Windows platform.
The bugs and glitches in the current GTK releases (3.22.4-ish) make floating utility windows basically unusable, to the extent that I don't want to ship with the feature enabled on Windows MyPaint right now
- Vertical offsets between where you click and the effect it has in the subwindow
- The thing with the crashes and the close button(s) is related, but not a showstopper
- Total inability to resize the floating window properly. Ugh.
Thankfully, the tools still work when they're docked in a sidebar, and you can drag them around still (with some careful thought, on Windows, because the little floating preview of the tab you get when dragging? Welded to the top left of the screen. gah!)
(And it all works so well in Linux... )
(On the Windows GTK front, I did just kill off a load of upstream tablet bugs though. Things are moving along, albeit a little slowly)