Files: gui/main.h, gui/undohistory.c
The Undo History window shows the undo history items of a tlo or the universe (the “global undo history”). Similarly to the Universe Browser window, it does not keep any copy of the data; it uses a GtkTreeModel object which provides the interface between kernel structures and the GtkTreeView widget. This is described in The GtkTreeModel Interface for Internal Structures.