Hello,
This is not an issue with Canvas (Working Example: http://themes.semicolonweb.com/html/canvas/index-shop.html) but with the Custom CSS Codes you have added to the style.css File and your Page. The Top Bar z-index have been modified which is causing this issue. You will need to reinstate this to defaults and add all your Custom Codes to css/custom.css File according to the instructions in the Documentation > Start > Customization Section to make sure that your Customizations are handled correctly.
Also unfortunately, we currently do not provide Customization Support according to the Item Support Policy: https://themeforest.net/page/item_support_policy so you will need to Code this manually or Hire a Freelancer to help you out with this.
Hope this Helps!
Let us know if we can help you with anything else or if you find any further issues.