Tested with Qt Design Studio 3.8 built with Qt 6.3.2 (snapshot 3)

If the Material Browser -window is small, you cannot drag-n-drop the materials at the bottom of the Material Library to the Material -field, because you cannot use mouse scroll to go back up, while holding the material. Only way to add materials from the bottom by dragging is to drag the material to the 3D -view or by expanding the Material Browser -window, so the Library -field is visible. This happens with both kits 5.15 and 6.3