Scroll bar should be visible 100% of the time

[Botspot]

Some users are not noticing that the list of operating systems is longer than the height of the window.
Yes there is a scroll bar, but it's invisible until you (1) start scrolling, or (2) happen to know there is a scroll bar there and move your mouse over it.

There's at least one documented instance of this confusing a user: https://www.raspberrypi.org/forums/viewtopic.php?p=1660762#p1660762

[lurch]

Very similar to #32