How to force position absolute with 100% width to fit into parent div with padding?

Just set left: 20px; and right: 20px; and remove width: 100%

Live Demo

.box2 {
    position: absolute;
    padding: 50px 0;
    color: #000;
    background: #fff;
    left: 20px;
    right: 20px;
    border: solid thin #06F;
}

or add left: 20px; and the calc function width: calc( 100% - 40px )

.box2 {
position: absolute;
width: calc( 100% - 40px );
padding: 50px 0;
color: #000;
background: #fff;
left: 20px;
border: solid thin #06F;
}

live Demo

If it has to be responsive, you could add the the padding as a margin and then use calc for the width:

.box2 {
    position: absolute;
    width: calc(100% - 40px);
    left: 0px;
    padding: 50px 0;
    margin: 0 20px;
    colour: #000;
    background: #fff;
    border: solid thin #06F;
}