Hide preview pane + new document = Only preview pane (MacOS)

I've been using MacDown for at least two years and love the simplicity and looks of it very much!

I've experienced this problem for a pretty long time and stopped using the feature, though I would really love to be able to use it properly.

I mostly use the editor-pane (with the color formatting) to write notes and have "Untitled" documents open before I copy/paste them into Docs or save them otherwise.

This means that I only use the Preview pane when I want to copy the formatting over to some place that does not support Markdown.

## The problem
When I hide the Preview Pane and create a new document (`cmd` + `N`) I **only** see the preview pane and no editor pane.
This is exactly what I don't want or expect.

Reproduce the issue in MacDown on macOS by hiding the Preview pane and creating a new document with cmd+N. Trace the new-document and pane-visibility behavior; done means the editor pane remains visible and the Preview pane stays hidden after creating the document.

Written by the indexing model from the issue text.