Shiftnav on mobile is causing jump and zoom out

Hello support,

When I’m on mobile and I click the shiftnav toggle, my site zooms out. Instead of the menu pushing the site aside and displaying full width, it splits the screen in half and doesn’t take up full height. Here’s a screenshot: http://howarddesign.com/wordpress/wp-content/themes/uncode-child/img/shiftnav.PNG

Could someone guide me on how to fix this?