Make sure that you clear your cache and see if that helps to fix it.
Sometimes when I am scrolling on the forums (both with mousewheel and/or sliders), black squares will appear at random on the screen, like in the image below. It doesn't always show them, but it is frequent enough.
Browser: Brave (Chromium-64)
Make sure that you clear your cache and see if that helps to fix it.
If you right-click on one of the black squares and select "inspect element" (or something similar) then it should show the html that's causing it -- post a screenshot of the html & element info.
Alternatively, compare it to what's shown by "inspect element" for something just outside the black box.
If there's nothing odd and/or obvious that shows up in inspect element then I'd probably speculate that it's an artefact caused by you graphics card playing up.
This is a known bug in and will be resolved in a browser update very soon.
Not something SbS can do anything about.