Since accessing the internet (and sometimes even the Web) is often a part of an IT job, the "one thing" approach is pretty helpful. Keep in mind what are you doing. If you're reviewing a pull request, you only need the Github page, and not the rest of the pages on the WWW.
Also, timeboxing things helps. It's hard to work on one thing indefinitely, it can even induce panic. But if you're using something like the pomodoro technique, you know that when you're feeling a craving to switch, you have, say, 27 minutes left until the next break. It's observable and manageable; you can concentrate during this time, knowing that a relief is nearing. But the relief (like going for more tea / coffee / etc) is also timeboxed, so you don't get distracted forever.
Also, timeboxing things helps. It's hard to work on one thing indefinitely, it can even induce panic. But if you're using something like the pomodoro technique, you know that when you're feeling a craving to switch, you have, say, 27 minutes left until the next break. It's observable and manageable; you can concentrate during this time, knowing that a relief is nearing. But the relief (like going for more tea / coffee / etc) is also timeboxed, so you don't get distracted forever.