Scrollbars in Design View have wrong sizes and positions

Libraries and Properties Editor scrollbars in Design View are broken: they occupy around 90% of their scroll area, centered inside of it and are not moving. Clicking or dragging them moves view port to the beginning of it's content.
Some other scrollbars, like Form Editor, Projects, Navigator are working correctly.

Win10, Build 48 (Qt6.2 based). Build 41 (Qt5.15 based) is working fine.