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

• Any manual on cell grouping management?
• Cells[] ignoring multiple style cells
• NotebookImport ignoring multiple style cells

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.