How can I expand a child div to 100% screen width if the container div is smaller?

The parent element of the whole page is a centered div limited to a max-width of 960px. All other elements on the page are children of that parent div. The simplified structure is the following:

<div id="parent">
  <div id="something"></div>
  <div id="wide-div"></div>
  <div id="something-else"></div>
</div>

While the parent div shouldn't expand beyond a width of 960px, the div I called "wide-div" here should fill the entire width of the screen. It contains a single image that is wider than the 960px, and it should set a different background color for the entire width of the screen.

I can't easily take that div out of the parent div, it would mess up other parts of my layout and it would make the whole thing rather awkward.

I found a few tricks on how you can achieve this, but none seemed to fit my requirements. My design is responsive, or at least I'm trying to achieve that. The tricks I found relied on knowing the size of the involved elements, which is not fixed in my case.

Is there a way to expand the inner div to the full screen width in a responsive layout?

Solution

You can set the width based on the vw (viewport width). You can use that value too using the calc function, to calculate a left-margin for the div. This way you can position it inside the flow, but still sticking out on the left and right side of the centered fixed-width div.

Support is pretty good. vw is supported by all major browsers, including IE9+. The same goes for calc(). If you need to support IE8 or Opera Mini, you're out of luck with this method.

-edit-

As mentioned in the comments, when the content of the page is higher than the screen, this will result in a horizontal scrollbar. You can suppress that scrollbar using body {overflow-x: hidden;}. It would be nice though to solve it in a different way, but a solution using left and rightlike presented in Width:100% without scrollbars doesn't work in this situation.

-edit 2021-

Another work-around for the scrollbars, which may be acceptable or not depending on your situation:
By making the green div a little bit smaller, say 20px, you can keep a bit of space for the scrollbar. Half that reserved width can be added to the margin, to keep the wide div centered:

#wide-div {
  width: calc(100vw - 20px);
  margin-left: calc(-50vw + 50% + 10px);

div {
  min-height: 40px;
  box-sizing: border-box;
}
#container {
  position: relative;
}
#parent {
  width: 400px;
  border: 1px solid black;
  margin: 0 auto;
}

#something {
  border: 2px solid red;
}

#wide-div {
  width: calc(100vw - 20px);
  margin-left: calc(-50vw + 50% + 10px);
  border: 2px solid green;
}

<div id="container">
<div id="parent">
  <div id="something">Red</div>
  <div id="wide-div">Green


<br>Green
<br>Green
<br>Green
<br>Green
<br>Green
<br>Green
<br>Green
</div>
  <div id="something-else">Other content, which is not behind Green as you can see.</div>
</div>
</div>