blue highlight box

a blue highlight box has suddenly appeared on the desktop screen on my 2017 MacBook Pro, and I can't get rid of it. It seems to be disrupting functions, such as I can no longer turn off the machine

Please can you help?

Does the blue highlight box do anything if you engage it somehow?

Did an Accessibility box get checked?

>System Preferences>Accessibility

You can force quit the Mac by holding the power button for ~8-10 sec. Does the restart resolve anything?

To trouble shoot further you can:

Try a SafeBoot  https://support.apple.com/en-us/HT201262

Takes noticeable longer to get to the login screen, does a 5 minute disk repair before it fully boots up, and certain system caches, dynamic loader cache, etc. get cleared and rebuilt.

This test will tell you if third party interference; extensions etc are not loaded in safe boot mode.

Test issue in another user (or guest user) account  https://support.apple.com/en-us/HT204443

This will tell you if it a universal issue or isolated to your user/admin account. 

Press and hold down the Power button until the computer shuts down. Wait 15 seconds then reboot.