Hmm, will look into this one. Have some idea what it might be!
Ah this is one of your extensions / plugins adding a div to the page. and my CSS defaults setting it in a particular way...
Extensions that add these kinds of elements to webpages are annoying, making assumptions about styling
I also have a weird div covering half the viewport (vertically): https://imgur.com/a/vJqWmtO
edit: the div covering half the viewport also occurs on nightride.fm