My website has sections with a background color applied. However, when the browser window width is less than the page width, a strange issue occurs - the background color only covers a portion of the elements equal to the window width starting from the left.
I have minimized the HTML and CSS to showcase this problem. I have tested it on IE11 and FF42.0.
h3 {font-size: 3em;}
.widthContainer {width: 960px;}
.test {background-color: #3870d0;}
<div class="test">
<div class="widthContainer">
<h3>The background color won't cover the whole heading!</h3>
</div>
</div>