Fixed context selector pane moving body

[RunDevelopment]

This fixes a visual bug for the node context selector. If the window height was too small, the node selector sometimes would not fit and cause the body to scroll.

I fixed this by

1. Making it so that the body can never scroll. and
2. Making sure the context selector always fits by giving it a max height of 50% viewport height.

As a minor design change, I also removed the weird padding at the top (and bottom) of the context selector. That one always bothered me, so I took the chance to remove it.