Divi mobile header (menu) keeps shifting

What I see on my iPhone is that on load the full set of menu links is displayed. It stays like that until you start to scroll the page at which point the menu switches to the three-lined hamburger mobile menu button.

I think this is because Divi's JavaScript is being deferred until the user interacts with the page. It looks to me like it might be done using the Flying Press plugin's Delay JavaScript Execution feature:

https://docs.flyingpress.com/en/articles/11406748-load-third-party-scripts-on-interaction

Since Divi initially displays the full menu, but then uses JavaScript to detect mobile / display the mobile menu button, this feature delays this from happening until the user actually touches the screen.

I think the solution would be to exclude Divi's scripts from Flying Press' Delay Javascript Execution feature. I think you probably need to exclude the following script:

/wp-content/themes/Divi/js/scripts.min.js

(possibly others too)

Alternatively, disabling that feature entirely should solve this issue, though of course you lose the benefits of deferring scripts which don't need to run until the user interacts with the page.

Hope that helps, but just shout if that exclusion isn't enough, or if this doesn't seem to be the issue you're dealing with.