uses
  Math;

type
  TStringGrid = class(Vcl.Grids.TStringGrid)
  protected
    procedure SelectionMoved(const OldSel: TGridRect); override;
  end;

implementation

{ TStringGrid }

procedure TStringGrid.SelectionMoved(const OldSel: TGridRect);
var
  I: Integer;
begin
  // invalidate selection changes
  inherited;
  // invalidate all the fixed columns
  for I := 0 to FixedCols - 1 do
    InvalidateCol(I);
  // invalidate all the fixed rows
  for I := 0 to FixedRows - 1 do
    InvalidateRow(I);
end;

procedure TForm1.StringGridDrawCell(Sender: TObject; ACol, ARow: Integer; Rect: TRect; State: TGridDrawState);
var
  Grid: TStringGrid;
begin
  Grid := TStringGrid(Sender);

  if (gdFixed in State) and (InRange(ARow, Grid.Selection.Top, Grid.Selection.Bottom) or
    InRange(ACol, Grid.Selection.Left, Grid.Selection.Right))
  then
    Grid.Canvas.Brush.Color:= $00FFDE9B
  else
    Grid.Canvas.Brush.Color := clwhite;

  Grid.Canvas.FillRect(Rect);
  Grid.Canvas.TextOut(0, Rect.Top + 4, Grid.Cells[ACol, ARow]);

  if gdSelected in State then
    Grid.Canvas.DrawFocusRect(Rect);
end;