I have found a workaround, quoted from the steam-de project's issue tracker:
Workaround
The black box only appears when the mouse is left hovering on the upper left corner. After starting Steam, moving the mouse away from the area, then changing focus (oddly enough, one has to alt+tab and then hit tab again while still holding alt [perhaps this makes it redraw?]) and the ugly box goes away!!!!
Same problem.
Hello, is anyone still experiencing this issue with an up to date system?
Closing pending feedback.
Nothing extracted yet.
If you open Steam with steam -bigpicture, as soon as the main screen shows, steam loses focus, when you alt+tab again to get focus on the top left corner of the window looks broken, and the mouse cursor changes.
This was originally posted as a steam-login issue here: https://github.com/thor27/steam-login/issues/8 but this actually happens when using steam with any DE that I tried.