Not sure you have seen this before. Sometimes, I scroll the web page with the mouse wheel and the web page scrolls down creating a gap above the black bar on top of the screen. Everything scrolls down, the black bar, menu, etc. If I could send you a screenshot would be ideal. This is the website I’m referring to: https://mclinteractive.com/
We checked the site and its working pretty well. Please check our site demo (https://www.themehorse.com/preview/inc/) once whether it is also having same kind of issue on you or not? If it worked well then your code customization might have affected it.