Anyone know of a single-pane Markdown editor for MacOS Just looking for an markdown editor that is one pane, and will let me toggle back-and-forth between "edit mode" and "display mode" with a keyboard shortcut.

I really how the cells in python's Jupyter notebooks can be toggled back and forth between markdown content, and want to duplicate that behavior.
Check out MacDown, it is free and although it has possibility to show edit both panes, you can close one pane using Shift + Cmd + H and then you can switch between edit and display mode using Shift + Cmd + E.

Hope it helps!