For the left div, you have:
a) Used position: absolute and bottom:0 so it is using all the available height anyway. So there is nowhere to expand into.
b) Set overflow: hidden. That tells it NOT to put up a scroll bar when the content exceeds the available space. So it doesn't.
c) Set too many parameters. E.g. If you set top and bottom, it is pointless also setting height. You should only set two out of these three, depending upon how you want the div to respond to different display heights.
d) If you want to set top to 15%, it is inappropriate to set height to 100%! Set it to 85%. Or, if you want the transparent header to overlap the left div, set top: 0 and height 100% (or bottom: 0).