I'm not sure how to describe this problem, but I have a website in wordpress that uses those menus that have reduced height when we scroll down, the problem is that the menu is getting reduced already in loading the page and after a few 2s it returns to normal.
It is as if the page were already loaded with 1px scroll down and after 2 seconds scroll up.
Load like this, what it looks like when it scrolls down:
Andafter2secondsitgetsnormal: 
What can be happening? I think it might be some css or script error, but how do I find this? The site is published, but I do not know if you can post the link here.