Odd black lines surrounding finder

[NerdsWithSwords]

For some reason my computer lagged for a few seconds when changing windows and these black lines surrounded my finder. It doesn't show up around any other window, just the finder. I've tried relaunching it, but it didn't change.

Any help is much appreciated!

 

[RedCroissant]

For some reason my computer lagged for a few seconds when changing windows and these black lines surrounded my finder. It doesn't show up around any other window, just the finder. I've tried relaunching it, but it didn't change.

Image

Any help is much appreciated!

Did you only try relaunching it, or did you also try shutting down and then restarting the machine?