Fixing intensely/exclusively on one task is called Perseveration.
It is not necessarily a part of ADD or ADHD.
I don't have ADD or ADHD, but I do have Perseveration.
In case anyone has not noticed yet.
It has helped me immensely, professionally. At one job one of the ladies called me 'Bulldog', because once I was given a problem I would not let go of it til it was solved. I just cannot stop until I've completely analyzed something and found the solution, even after I find the solution I want to look for more problems in it, LOL.
If there are dozens of levels to a problem most people have had it by the time they get down 2-3 levels. Me, I'm just getting warmed up by then. My one boss said, 'she can build you a rocket ship that will fly you to the moon, but don't ask her to take out the garbage' - he meant stop me to do more of a side task. I look up with a dazed look and say, 'HUH?' It usually takes me several minutes to 'come up for air', resynch and figure out what the person is talking about. 'The what? The shipping bills project? What shipping bills?'
Once at a meeting a problem came up. My boss was there and quipped, 'Give it to Wels, she will have it by the end of the meeting'. I took the writeup, and I started reading, in a few minutes, my boss said, 'Look, there's steam coming off the top of her head'. I didn't get it by the end of the meeting, so when the meeting ended he took ahold of my sleeve and led me down the hall while I've got my head buried in the writeup, going, 'ok, then they hand off the shipment to....' Everyone was walking down the hall by us laughing.
Ever notice how long my posts are? Hm? Hm?
I just gave a ton of examples, and then came back and deleted them. Very, very painful to not include a dozen examples. It makes me nervous, LOL. It's not finished.