bugs - Cell @ CellGroupData[... in Cells family

Bug introduced in V10.4 or earlier and persisting through V11.3

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Background

This question originates from two already posted questions updated with further doubts I've faced. I deleted one and merged content here.

Those issues are closely related, hard to separate and probably could be caused by the same internal features so I decided to make one question

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CellGroupData assumptions

First of all, according to tutorial/TheStructureOfCells a cell of the form Cell[CellGroupData[...],...] is a valid Cell so in my opinion we should expect consistent behavior.

CellGroupData second argument allows to specify which cells are visible. It could be Open, Closed, {cells indices}, or even {} to show no cell.

That is not the case in general:

Issues

1. 
   

   Inner CellGroupData expression are transparent to parent's visibility spec:

   
   

   CellGroupData[
   
      { Cell[...] 
      , Cell @ CellGroupData[...]
      , Cell[...] 
      }
    , {2}
   ]
   

   
   

   I'd expect the structure above to show only and exactly what Cell @ CellGroupData[...] contains.

   
   

   It is not the case, the outer CellGroupData can't handle nested structure this way and just goes into deeper CellGroups which is unintuitive and contradicts CellGroupData documentation.

   
   

   Try yourself

   
   
   

   Notebook[
       { Cell @ CellGroupData[
             { Cell["1", "Text"] 
             , Cell @ CellGroupData[
                  {Cell["2-1", "Text"], Cell["2-2", "Text"]}]
             , Cell["3", "Text"]        
             }
           , {3}
         ]   
       }
   
     , CellGrouping -> Manual
     , ShowGroupOpener -> False
   ] // NotebookPut
   

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2. 
   

   Visibility spec problem for single cell CellGroupData

   
   

   But if the group contains only one cell the {} visibility spec does not work:

   
   
   

   Notebook[{
      Cell @ CellGroupData[{ Cell["Group 1", "Chapter"] }, Open],
      Cell @ CellGroupData[{Cell["Group 2", "Chapter"]}, {}]
      },
     CellGrouping -> Manual
   ] // NotebookPut
   

   
   

   Add a second cell to the second group and it will disappear!

   
   

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3. 
   

   Cell @ CellGroupData as a first cell in Cell @ CellGroupData

   
   

   When the first cell in a group contains another CellGroupData then the parent CellGroupData is confused.

   
   

   Not only {} will still show whole group but even giving specific {1} cell position fails.

   
   

   Notebook[{
      Cell @ CellGroupData[{ Cell["Group 1", "Chapter"] }, Open],
      Cell @ CellGroupData[
          { (*Cell["","Text"],*) (*uncomment this cell and everything is ok*)
   
            Cell @ CellGroupData[{
                Cell["Group 2", "Chapter"], Cell["Group 2", "Text"]
            }]
          , Cell["text", "Text"]
          }
        , {2}
      ]}
    , CellGrouping -> Manual
   ] // NotebookPut
   

   
   
   

   

   
   

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4. 
   

   Deletion of CellGroups in Editable -> False notebook

   
   

   You can delete each cell one by one, even those from the cell group. But if you try to delete after selecting group bracket you will get a beep and won't be able to delete it.

   
   

   CreateDocument[
    { Cell @ CellGroupData[{
   
         Cell["1", "Text"]
       , Cell["2", "Text"]
      }]
    , Cell["2", "Text"]
    }
    , CellGrouping -> Manual
    , Editable -> False
   ]
   

   
   

   Whatever editable means, this is at least inconsistent.

   
   
   

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Further reading

related to cell structure

Questions

How to understand cell groups behavior? Seems detached from 'cells framework' for me. Not to mention a minimal documentation.

Classification of faced 'issues' will be appreciated.

Answer

[...] Thank you for consolidating all of the issues to a single StackExchange article. I have confirmed the behavior you are reporting in each of the four issues. I will add a link to this StackExchange article in the developers report I originally filed. [...]

[CASE:3918002]