Unable to scroll in trade window

[thomotron]

Description

None of the scroll bars in the trade window actually scroll, they're all static. Trying to use the scrollbar by dragging, clicking further along, and using the scrollwheel all do nothing.

Steps to reproduce

1. Open a trade
2. Fill one of the panels (available items, our offer, or their offer) with enough items to render a scrollbar
3. Try to scroll the panel

[thomotron]

Looks to be related to changes made in VerticalScrollContainer that internalised the previous scroll position and defaulted it to the top. Since the trade window is still generating the container each frame, it will always default to the top and never update.