Are you on a PC with Windows O/S software? If using Windows, at the upper right of the screen do you see [a short line, a single/double box, and an X]?
If single box is seen, click on that icon to expand the window to the full width of the monitor (double box should appear when at full screen display)
On your monitor, you would be seeing use of 1280 pixels, so the black sides should then be visible.
What browser are you using?
Windows 10
Google Chrome
This is what I see (upper right hand side) when looking at my blank window screen - is this what you are asking about?
You lost me on this question/statement above




