Hi mrplants,
Welcome to Apple Support Communities. We understand that you're having an issue with a line appearing on your menu bar in Safari. We'd like to help.
Does this line disappear when you add a new tab to that window? Does it happen when you're using a private browser? To check, with Safari open, click File > New Private Window. Do you see the line there as well?
We'd like to have you try to boot into safe mode to see if the same thing happens.
How to use safe mode
Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:
Apple silicon
1. Shut down your Mac.
2. Turn on your Mac and continue to press and hold the power button until you see the startup options window.
3. Select your startup disk, then press and hold the Shift key while clicking “Continue in Safe Mode.”
4. Log in to your Mac. You might be asked to log in again.
Intel processor
1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac.
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.
Then, continue here to determine the best next steps after you determine if this happens in safe mode or not. How to use safe mode on your Mac
Let us know how that goes.
Cheers!