I have created a layout in CSS/HTML with the following structure:

Navigation in the footer scrolls the content horizontally to show two different screens. The screen content is fixed height and vertically centered and the layout always fills the entire window.
Because my content has a fixed height, I need the browser to display a vertical scrollbar when the window reaches a certain height.
I also have a couple of position:fixed <canvas> overlays that get cropped by the <html> and <body> elements which are set to height:100%; width:100%.
I hoped that by setting min-height:700px on the <body> selector I could get a scrollbar to appear when the window got too short. This doesn't seem to be work.
How can I make this website generally full-screen but scroll when below a minimum size threshold?