Note: The UndoUnit.Merge method is only invoked when 2 or more new UndoUnits are added to the history. If an Undo or Redo operation is performed, the method will not be invoked until after the next non-undo/redo operation is performed.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Server 2012, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2