If an undo group is open, gets the group descriptor of the current top-level
undo group.
If no undo group is open, gets the group descriptor from the previous undo group.


The group descriptor can be used to join adjacent undo groups:
use a group descriptor to mark your changes, and on the second action,
compare LastGroupDescriptor and use StartContinuedUndoGroup(Object) if you
want to join the undo groups.
Assembly: ICSharpCode.AvalonEdit (Module: ICSharpCode.AvalonEdit.dll) Version: 5.0.0.4238