weird UI 'jump to content' button thing

There's this odd thin 'scroll bar' looking line at the top left of the site page.

That's the top of the pic.

When I click it a 'Jump to content' button appears.

That's the bottom of the pic.

Here's the code of the button:

NVM, can't figure out how to retain the html in a code block. >Jump to content

Anyone else notice this?

OS: Linux Browser: Firefox