Button disappearing behind menu on mobile

Hi there, I am currently experimenting with adding a floating button that's fixed at the top of the page which takes you back to the previous page on certain pages of the website (only experimenting on an old page that's not live at the moment...) I have an issue that the button stacks in front of the menu when you open it, which is really impractical. Instead, I'd like the button to stack behind the menu, so that if you click the menu, the options are all visible, not obscured by the back button. Any ideas? 

My best guess is that you have set a high z-index for the button.

Can you share your website URL? and the code you are using for the button?

Ah yes, that's the issue for sure! It's set at 9999. I don't know what number would make it above the images on the pages, but not above the menu, any idea?

Not sure, adjust the value down whilst looking at the menu until it drops below, trial and error tends to be a good approach here.