black strip on the safari screen

Hi there onat66,

If you are seeing this horizontal black line in Safari on your Mac, does it stay in this area of Safari or does it appear in other areas as well? Do you see this line in other apps or on the Desktop of your Mac? This information will allow us to isolate what's happening more.

Also, let's ensure your Mac is fully updated since this installs feature improvements and bug fixes that resolve unexpected behavior like this. These steps from Update macOS on Mac show how to back up and update the Mac:

Before following these steps, it's a good idea to back up your Mac.

1. From the Apple menu  in the corner of your screen, choose System Settings or System Preferences.

* If you chose System Settings, click General on the left side of the window. Then click Software Update on the right.

* If you chose System Preferences, click Software Update in the window.

2. Software Update then checks for new software. 

* If Software Update finds new software, click the button to install it. The button might be named Update Now, Upgrade Now, Install Now, or Restart Now, for example. You might then be asked to enter your administrator password.

* If Software Update says that your Mac is up to date, then no new software is currently available for your Mac model. Software Update shows only software that is compatible with your Mac.

Cheers!