Open two copies of the same notebook

Question

Is there a way to open one notebook in two (or more) different windows? This can be helpful when working with a long notebook and one wants to edit one part of it while looking at a different part. Otherwise, you have to jump back and forth...

Edit: Per @Guillochon comment, splitting the window where the notebook is open is indeed a nice option.

Edit 2: Having a secondary "read-only" window can also be a nice compromise.

Answer 1

Here's a rather unsophisticated approach. The code below creates a toolbar in the current notebook, with buttons to create, refresh and close a replica notebook. The replica is given a gray background and cannot be edited or evaluated.

I tried to do something with Dynamic to automatically refresh the replica notebook, but without success.

Update

Here is a neater implementation. The code below creates a palette (which can be installed using the menu: Palettes/Install Palette) containing a single button to create a duplicate of the active notebook. The update button for the duplicate is now located in the duplicate itself, leaving the original notebook unchanged. The duplicate is closed using the normal window close button, but it has "ClosingSaveDialog" set to False so you won't get a save prompt.

CreatePalette[Button["Duplicate Active Notebook",
   NotebookPut[NotebookGet[InputNotebook[]] /. 
   {Rule[DockedCells, _] :> Sequence[], 
    Rule[WindowMargins, _] :> Rule[WindowMargins, {{0, Automatic}, {0, Automatic}}], 
    Cell[x___] :> Cell[x, Evaluatable -> False]}, 
    Background -> GrayLevel[0.95], Editable -> False, "ClosingSaveDialog" -> False, 
    DockedCells -> With[{sourcenb = InputNotebook[]}, 
      Cell[BoxData[ToBoxes[Button["Update", 
          SelectionMove[InputNotebook[], All, Notebook]; 
          NotebookWrite[InputNotebook[], 
           NotebookGet[sourcenb] /. Cell[x___] :> Cell[x, Evaluatable -> False]]]]],
       "DockedCell", CellContext -> Cell]],
    WindowTitle -> "Duplicate of " <> AbsoluteOptions[InputNotebook[], WindowTitle][[1, 2]]];
   SetSelectedNotebook[InputNotebook[]]], WindowTitle -> "Duplicate"];