SetOptions[EvaluationNotebook[], DockedCells -> Cell[#, "Output", CellMargins -> 0]] &@
 BoxData@ToBoxes@
  With[{GetTOC =
     Cases[NotebookGet@SelectedNotebook[],
      Cell[name_, style : "Section" | "Subsection" | "Subsubsection", ___] :> {style, name},
       Infinity] /. {"Subsubsection", x_} :> x[] //.
     {x___, {"Subsection", y_}, z : Except[_List] ...,
       w : PatternSequence[{_, _}, ___] | PatternSequence[]} :> {x, y[z], w} //.
     {x___, {"Section", y_}, z : Except[_List] ...,
        w : PatternSequence[{_, _}, ___] | PatternSequence[]} :> {x, y[z], w} &},
    DynamicModule[{TOC = GetTOC[]}, Row@
      {Button["Update", TOC = GetTOC[]],
        Dynamic@Refresh[" " <> StringJoin @@ (ToString[#] <> "." & /@ #) <>
        " " <> If[Length[#] == 0, "",
         Head@Catch@Fold[If[Length[#] >= #2, #[[#2]], Throw[#]] &, TOC, #]] &[
      CurrentValue[NotebookSelection[], {"CounterValue", #}] & /@
{"Section", "Subsection", "Subsubsection"} /. {x___, 0 ...} :> {x}],
     UpdateInterval -> 1]}]]

Button["press", Print @ FrontEndExecute @ FrontEnd`CellInformation[NotebookSelection[]]]

current = First @ Cells[ExpressionUUID -> "a4103240-2c81-4d64-90fc-deb0941ca4a0"]

PreviousCell[current, CellStyle->"Section"]

Dynamic[FrontEndExecute @ FrontEnd`CellInformation[NotebookSelection[]]]

Dynamic[{
    CurrentValue[NotebookSelection[], CellStyle],
    FrontEndExecute @ FrontEnd`CellInformation[NotebookSelection[]]
}]

SetOptions[
    EvaluationNotebook[],
    StyleDefinitions -> Notebook[
        {
        Cell[StyleData[StyleDefinitions->"Default.nb"]],
        Cell[StyleData["Notebook"],
            DockedCells->Cell[BoxData @ ToBoxes @ Dynamic[
            Row@{
                CurrentValue[NotebookSelection[],{"CounterValue","Section"}],
                Replace[CurrentValue[NotebookSelection[],{"CounterValue","Subsection"}],{0->Sequence[],n_->Sequence[".",n]}],
                ": ",
                Replace[Cell[a_,__]:>RawBoxes@Cell[a]] @ With[
                    {cell=First[Cells[ExpressionUUID->OptionValue[FrontEndExecute@FrontEnd`CellInformation[FrontEnd`SelectionObject],ExpressionUUID]],{}]},
                    Which[
                        cell==={},
                        "",
                        MemberQ[CurrentValue[cell,CellStyle],"Section"|"Subsection"],
                        NotebookRead[cell],
                        True,
                        NotebookRead[PreviousCell[cell,CellStyle->{"Section","Subsection"}]]
                    ]
                ]
            }
            ],
            TextAlignment->Center
            ]
        ]
        },
        StyleDefinitions->"PrivateStylesheetFormatting.nb"
    ]
]