Custom page size defaults to negative margins

This is a minor nit, but it caused some consternation until I figured out what was going on. I switched a score I had entered in A4 format, which was multiple pages, to a custom size of 10"x14". As soon as custom is selected the left and right page margins become negative. I didn't spot this at first. The effect is overlaid systems along the page edges. You need multiple pages to exist before switch to custom page size to see this effect. It applies to the 3.0.5 and the nightly build from Friday 26th April.

It would be a little more friendly if selection of a custom size didn't allow the margins to become negative.