Try it. Note that I have included only necessary changes to your examples.
body {
padding: 0;
margin: 0;
}
#content #left {
height: 100%;
position: absolute;
top: 0;
z-index: -1; /* Hide right border below #header */
box-sizing: border-box;
/*padding: 0 0 0 10px;*/
padding-top: 30px; /* Height of #header */
}
If negative z-index not working in older browsers, then use following style.
#header {
position: relative;
z-index: 1;
background: #fff;
}