I can reproduce this every time I run a search from the top search box in ForgeBox. Basically, when the search results show up, the menus in the upper left hand corner and right hand corner will momentarily render as all visible on the page for a split second before hiding. I'm not sure if it's related to the CSS taking a few ms to download and load but there has to be some direct styles we can apply to those elements to ensure they stay hidden as soon as they are rendered until the proper time that a user mouses over them and they need to show.
Here here is a screenshot that shows what the screen looks like for a brief second
We will move to another css framework soon, not worth spending the time.