Replacing cells with content from cells within the same column

Hello - here is an example of how this can be solved.  You can get the value of the nth row above, but this leaves the opportunity for retrieving the wrong value if there is an unexpected number of rows in the set above.  This solution fills the null values in column 2 with the value of the first row above which has the same column 1 value.

SCRIPT

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("bc2pEQAwDAPBXoQN8jhfLR7330YcA6GwBbqRGSoEEy6GFlqpHtopDZ3UCNWSfIkyGUw+xWbAvX4uFoMG9ws=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", Int64.Type}, {"Column2", Int64.Type}}),
    RowIndex = Table.AddIndexColumn(#"Changed Type", "Index", 1, 1, Int64.Type),
    #"Added Custom" = Table.AddColumn(
        RowIndex, "Custom", each
        [Column2] ?? List.Last ( Table.SelectRows ( Table.FirstN ( RowIndex, [Index] - 1 ), (x)=> x[Column1]=[Column1] )[Column2] ), Int64.Type
    )
in
    #"Added Custom"

RESULT