This is another of those little things that are right there and yet many people seem to not know about them.
There’s a little handle at the top-right corner of the code window:
When you drag it down, it will split your code window into two parts:
This is very useful when you want to look at two different places within the same code file (e.g. while examining one method, check another one declared earlier in the same file).
At any time, you can drag the splitter in the middle all the way to the top to go back to single view.