It looks like https://gerrit.wikimedia.org/r/#/c/189506/ didn't take into account the Main Page special formatting. If the main page includes something like...
<div id="mf-tfa"> == Header == </div>
...which is common, it will end up with a 100% high edit button on the side of the page (taking up the entire height of the window).
Also, I think this change needs approval from product and design before being deployed in stable mode.