I am using debeautify theme, when I am in mobile or desktop view I can see the sidebar/navigation menu. However, when I enter fullscreen mode It disappears. You can see it in the top left corner in the screenshots I have attached. Also, if it helps the URL for the website is www.gearheadgadgetstore.com.
Sorry, you are facing this issue, it would be my pleasure to help you.
Welcome to the Shopify community!
Thanks for your good question.
I have seen your issues it can be resolved through the custom code.
I have to check your All code files from where issues are ocurring.
let me know if you need any more help.