Open LoanApp sample schema.
Open a new schema
In the new schema, in Schema view, right-click namespace, Add > Import
In Import,
  expand By File, select newLoanApplication.xsd
  enter a prefix
  click OK
In Design view,
  drag/drop an element, press <CR>
  -> newElement; Undo is enabled
  click Undo
  -> newElement disappears; Undo remains enabled; Redo is enabled
  click Undo again
  -> Undo/Redo remain enabled
  click Undo twice more
  -> now newElement returns; Undo/Redo remain enabled
  click Undo
  -> newElement disappears; Undo finally disabled; Redo enabled
  click Redo
  -> newElement returns; Undo/Redo remain enabled
  ckick Redo
  -> newElement disappears; Undo/Redo remain enabled

Apparently Undo/Redo account for the Add > Import step as well, which explains
why Undo stays enabled after newElement is undone.  One can keep clicking on
Undo while not really observing anything UI-wise in the DV, hence the momentary
confusion...