Ask HN: App to challenge before switching windows

I have a problem maintaining focus avoiding the temptation to multitask. My idea: Lock me in the current programme. When I Alt+Tab or switch to another window, pop up a box asking me to type in a code (variable length, I can change in settings) before I can switch. That prompt should be enough - if I don't actually need what I was just about to do (hello Twitter) typing in 24 random characters is going to stop me. Whitelisting apps would be useful (e.g. If I'm in app X, I can switch to app Y without prompting) Anyone know of a tool like that for Windows/OSX? If not, hope someone can make a business from it :)