I find that I tend to have at least two files open side by side when coding. Maybe it's an OOP thing, but I rarely focus on just one file. Even when coding web stuff, I'll hop between HTML, an associated CSS file, and a separate javascript file all open at the same time so that I can see the interrelationships. I find the cost of context-switching between files too high when I work on a laptop with only enough screen real estate to open 1 file comfortably.