The editor scripts source code can be found in the package at the "editor/_source/" directory. It is "human readable" as well as commented. There are a lot of files there, organized by groups.
To maximize loading and execution performance, the production distribution code in compressed to fewer and smaller files, as described in "Performance". These files are found at the "editor/js/" directory.
Using the "_source" files
It is possible to run the editor using the original scripts in the "_source" directory. It is useful when developing or debugging the editor. It is quite easy to do that.
Using a special URL parameter
Just add the "fcksource=true" parameter (QueryString) to the URL of the page where the editor is loaded.
Using the "original" file
There is also a definitive way to tell the editor to use the "_source" files.
Just go the "editor" directory and overwrite fckeditor.html with fckeditor.original.html (remember to backup the first one).
To restart using the "compressed" files again, just recover the backed-up "fckeditor.html" file.