I locked my MacBook to clean the keyboard. While cleaning I pressed a number of keys in a random way while the system was displaying the login screen. At a certain point I noticed a light-blue double frame around my account icon and its name.
It wasn't user selection which is a square shadow behind the user icon (selectable with arrow keys). When I confirmed the selection and the screen changed to the password login prompt, the blue frame disappeared.
It was around the account I had been logged on to before switching to the login screen.
I tried lock the screen again and repeat the process, but could not get the blue frame to reappear.
The question is: what is it? And how to repeat the process?
I included two screenshots to show that the user selection is separate from the blue frame.