Hello 
That line is the Skip to main content link bottom. I can repro this if I change the Edge font size.
On 100% default font size it seems good.
However if I set this a bigger font size like 110% or more the Skip to main content button which is hidden above header will show up because it seems the browser font size changer isn’t respect things…
Seems after reload the page, things back to normal
Before reload 200%
After reload
But there is a font size changer in Discourse what you can use instead of browser’s one and you can use the device native accessibility features too. These will respects the elements.
Change text size in Discourse:
Go to the interface page where you can change the text size.