Screenshot leaves a brighter box onscreen until I reboot mac

Hello Whort, 

We understand that after taking a screenshot on your Mac, the shadowed area remains. We'd like to assist.

Here’s the article on how to use that feature: Take a screenshot on your Mac

If you are using Command + Shift + 4, which is how you make a screenshot of a specifically chosen area, does anything similar happen? With this method you’ll not see a shadow around the unselected area. 

Let’s try using safe mode to resolve the issue:

How to use safe mode on your Mac

If that does not help, please test the issue in a different user account:

Set up users, guests, and groups on Mac

Let us know the results.

Thanks.