Unexpected behavior of SelectionMove

Question

Bug introduced in 11.2 or earlier, partially fixed in 12.0 and finally fixed in 13.0 [CASE:3968507]

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^{(Cross-posted at Wolfram Community.)}

I think this is a bug. If someone can help to confirm it, I'll report it to Wolfram. I can selet previous cell group:

SelectionMove[PreviousCell[], All, CellGroup]

But I fail to do this like follows

Maybe you will say that it is a Cell not a CellGroup, but why I can do this:

Can anyone give a reasonable explanation? You can get the .nb test file by run

NotebookPut[Uncompress[First[Values[Databin["fVOforSX"]]["nb"]]]]

Answer 1

I notice that some changes in the behavior of SelectionMove occurred on the transition from version 12.3.1 to 13.0.0.

1. Changed the behavior of SelectionMove[EvaluationCell[], Previous, CellGroup]. Now it selects the previous CellGroup, even if between the previous CellGroup and the EvaluationCell[] present several cells which aren't a member of a CellGroup. Here is a screenshot of a version 13.0.0 Notebook right after applying the menu item Evaluation ► Evaluate Notebook:

   

   And here is what happens in version 12.3.1:

   

   The new behavior is more consistent. So the inconsistency described in the question is finally fixed in version 13.0.0.

2. In the same way changed the behavior of SelectionMove[PreviousCell[], Previous, CellGroup]. Now it selects the CellGroup before the PreviousCell[] even if PreviousCell[] is a member of a CellGroup itself. Here is a screenshot of a version 13.0.0 Notebook right after applying the menu item Evaluation ► Evaluate Notebook:

   

   And here is what happens in version 12.3.1:

   

So this aspect of the behavior of SelectionMove is finally fixed in version 13.0.0.

Answer 2

This indeed looks like a bug. Citing the Documentation page for SelectionMove:

SelectionMove returns $Failed if it cannot move the selection in the way you request.

In your example SelectionMove obviously fails but returns Null instead of $Failed what directly contradicts the Docs.

One workaround is to check whether the selection is empty (SelectionMove failed) and if so to attempt to select Cell instead of CellGroup:

SelectionMove[PreviousCell[], All, CellGroup];
If[SelectedCells[] === {},
  SelectionMove[PreviousCell[], All, Cell]];

Another way is to rely upon the working functionality:

SelectionMove[PreviousCell[], Before, CellGroup]
SelectionMove[EvaluationNotebook[], Next, CellGroup]

(instead of EvaluationNotebook[] one can use ParentNotebook@PreviousCell[]).