It looks like the very edge of an open window on the left my laptop screen. I can't drag it or right click on it. I can't see the top of it but the bottom edge of it is rounded like the corner of an open window. Checked running processes and rebooted but it did not help.

can u post a screenshot please?

And if you reboot, it's still there? That is odd. You might see what gets started at boot.