I'm looking for an easy-to-use, free source code comparison app for Windows, which will highlight differences side-by-side between two pieces of source code. Some apps get close to what I want, but are too restrictive by requiring you load in entire files and compare them in their entirety. Sometimes I just want to compare a section of my file, such as a single function, which may be in totally different locations in the two versions I'd be comparing, making it hard to find in both panes in large files. Basically, I'd like to be able to simply edit/copy/paste the content in both panes rather than have the restriction of using files. That way I can copy and paste one function into one pane and another into the other, editing/re-ordering as necessary.

(Note that I realize there are other comparison app recommendation threads out there, but I'm having a hard time finding a free app that isn't a strict file-to-file comparison app)

Thanks for any pointers or links, thanks!