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


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



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




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





  1. 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!








  1. 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


    enter image description here







  1. 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.






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]



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