Relatively Positioned Submenu and Background Banner Doesnt Fill Width of Screen

[lastpeony]

I am trying to position submenu div and a banner made from a background.

The problem is they are not filling width of screen.There are small white spaces on left and right sides.
Also in my real app,submenu div is appearing after a button click,so it must push below element down.I mean i can achieve 100% width if i make submenu absolute position,but then below banner div is not pushed down.

Here is the codepen with minimized problem:

https://codepen.io/anon/pen/roEQqo

Gif for better explanation:
https://i.imgur.com/TvPDHyE.gifv

[Shikkediel]

I think you’re referring to the default margin on the body element.

body {
  margin: 0;
}

[Author]

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