It's not, it's so you can see the graphic changes better.
Oh really? I totally wouldn't have thought that this is intentional as it felt like something isn't right since this only happens when you use the arrow keys.
When you type the fps limit manually e.g. the handheld console would not disappear for a short moment. That's why I was kinda sure I found a bug. Maybe consider removing the disappearing effect to avoid further confusion. I'm sure I'll not be the last person thinking this is a bug in the options menu.
Ah I get it now. Using the arrows on the fps limiter is the only way for the background to hide, while for the other fields, changing the value or using the mouse hides the background. Yes, that's a bug. I took a note of it.