Fixed Full-height Side Nav
Try to scroll this area, and see how the sidenav sticks to the page
Notice that this div element has a left margin of 25%. This is because the side navigation is set to 25%
width. If you remove the margin, the sidenav will overlay/sit on top of this div.
Also notice that we have set overflow:auto to sidenav. This will add a scrollbar when the sidenav is too long
(for example if it has over 50 links inside of it).
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