The undo and redo button only apply to changes made since the last moment you entered editing mode for a particular text area, since the last page load.

In other words if you cancel the editing of a text area, or go to a different page, and then re-enter the text area the undo/redo buttons will not recover any prior changes.

Button icon	Name	Description
	Undo	Undo the last change made within a text area in editing mode. All changes are lost once the editing mode for a text area is left.
	Redo	Redo the last "undone" change made within a text area in editing mode. All changes are lost once the editing mode for a text area is left.