bugs - Unexpected behavior of SelectionMove

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

_{Now SelectionMove returns $Failed when it cannot move selection in the requested way, but inconsistent behavior described in the question is still here.}

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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

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[]).