Side panels not dockable after being undocked and closed once
1) Undock a panel (such as Palettes or the Piano Keyboard).
2) Close the now floating panel window.
3) Re-open the closed (floating) panel window.
4) It is now not possible to re-dock this panel window, neither by dragging it into the desired position or double-clicking on it.
The only way to get the panel back into docked position is to reset MuseScore 3 to factory settings.
Windows 10, 64-bit.