When I'm trying to learn some (sequential) information on a piece of paper by heart, I put a colored sheet of paper as a cover on it and pull it down line by line.

Now I'm searching for software that can do this functionality in Windows XP or Ubuntu, alternatively it could be a plugin for Chrome/Firefox/LibreOffice. Basically it would just need to be an intransparent window that is fixed in a certain location, but only while a specific task has focus, I want to be able to normally open other tabs or software applications. 

I'm sure that something like this must exist, but somehow it is very hard to google for it. I would really appreciate it if anyone could give me a hint!