Fix odd rendering when splitter dragged to the top

[magland]

From Brian:

Also, I don't think this is a new behavior here, but just noting that some really odd rendering happens if you drag the separator of the output area to the top of the window:

Originally posted by @WardBrian in #47 (review)

[magland]

Just a note... we may want to eventually swap out this custom splitter for something maintained by a third party... as part of a larger overhaul.