Strange blue border around Selected window/link

Uncheck "Enable Full Keyboard Access" in System Preferences > Accessibility > Keyboard > Navigation.

Go to System Preferences->Keyboard->Shortcuts and UNcheck

"Use keyboard navigation to move focus between controls"

GarryKaye wrote:

I didn't try this because something I did in the previous recommendations the problem appears to have been fixed - last thing I did was the - Uncheck "Enable Full Keyboard Access" in System Preferences > Accessibility > Keyboard > Navigation

Told you. 😎

I had the same thing happen - turned off Switch Control setting in the Accessibility section and it stopped!

nicevillesteve wrote:

I had the same thing happen - turned off Switch Control setting in the Accessibility section and it stopped!

You do know this thread is about Macs and macOS and not iPhones? :)

Boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

Possibly in addition to what dialabrain said: go to

System Preferences->Accessibility->Display

and make sure that "Increase contrast" is UNchecked. Does that help?

Download and run EtreCheck, and share the report to the clipboard. Then open a new reply here, and press the button that looks like a printed page to get an Additional Text box big enough to paste the hardware and software configuration report here, and paste the report here.

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

So you are saying that you had this already unchecked, and the issue persists?

Please follow Old Toad's advice, and let us know if the problem happens in Safe Mode, and if it happens when you then restart normally.

One avenue to contact Apple is via Feedback - macOS - Apple.  However, if you're already have a case opened you may want to call using the number at the bottom of this page and ask for Customer Support of Engineering. Give them the case no.

This mail thing is unrelated, it is how Mail recognizes what it calls "Conversations".

You can change the viewing options in the View menu. I prefer to keep "Organize by Conversation" checked, as it groups all the messages from the same conversation as one.