"copy category"function is very helpful when creating experimental versions of new classes - it allows a very quick class duplication.
So, instead of modifying any existing class, it is better to create a new one and
copy all methods over from the original one.
Then test & debug the new class until perfect.
When done, you can override the old one with the class
This approach is especially suggested when you modify classes which are used by
the system itself, since bugs in those may lead to subtile errors or even crashes.
(just consider modifying
ScrollBar to make it err somewhere; since the Debugger
uses scrollBars too, it may have problems coming up. As a result, you
might be forced to quit Smalltalk and restart your session).
Since the source information in a method is simply the filename+filePosition, this information is wrong after such a change.
A recompile (using stc) followed by a relink of the Smalltalk executable, should give you a version with correct (up-to-date) source information. (however, the demo version does not include stc - here you have to undo any outside world editing, and only change methods from within the SystemBrowser).
The commercial versions include a sourceCode manager, which is able to access a classes
sourcecode from the source resository - it does not need any source files.
Here, the above problems are unlikely to occur (unless someone changes the classes version identification manually ...)