I’m having this problem also but there is a workaround.
On the widgets page, hover your mouse over “Available Widgets” and you’ll see a small up arrow appear to the right. It’s not obvious, but this is an accordion menu and works the same as the widgets do – they open and close. Click the up arrow and the menu will contract. The “Inactive Widgets” menu heading will now appear directly below the screen and you should now be able to move your widgets freely into that section.