A side note:
I noticed there is getUndoRedo() method in TopComponent. However, I have one
DiffTopComponent that can contain many editor panes with editable documents each
one with its own UndoRedo instance ... and I am not sure that delegating will be
of any help here.

Update: Undo delegation works with one exception: I have no way of telling the
UndoAction that my UndoRedo support has changed for my TopComponent.

Re. delegation. Create a wrapper UndoRedo, delegate in it, when delegate 
changes, fire change event. Should be simple, UndoRedo is an interface.

Re. actual problem of getting UndoRedo for a Document, I like more an more to 
use document.getProperty("org.openide.awt.UndoRedo") and making sure that our 
CloneableEditorSupport documents have this property correctly assigned.

Deleting dependency and tracking independently.

Reassigning to new module owner mslama.