This is the sort of thing that all Smalltalkers - myself included - take for granted - that the code pane of the browser is just a special purpose workspace. It's interesting to note how unusual that is for developers who don't use Smalltalk much:
Notice that there is no UI for creating new methods! There's a strange and subtle shift in UI design thinking in effect here. Instead of the purpose of the code pane being to "edit some little snippet of code already associated with the current class", thereby necessitating UI for creating new methods, its purpose is to "interpret any submitted piece of code in the context of the current class", thereby permitting both definition of new methods and updates to existing methods.
This is, to my way of thinking, a wonderful thing about Smalltalk, but: it is the sort of thing that throws newcomers off at first.