Right editor won't expand very far in vertical split

I can’t figure out if this is my fault or not, but I’m looking at a view of two cork boards, split vertically. No matter what the contents, or even if I switch to outline or text view, the right editor refuses to expand beyond a fairly narrow width, while the left editor remains about 3.5 times wider.

Horizontal splits do not exhibit this kind of issue, I can drag the split to just about any size I like.

Any suggestions on what settings might affect this?

Beta 0.0.46
Project created in Mac Scrivener, shared on Drop Box.