[pirsddd]

Разобрался.

Код:

unit StringGrids;

interface

uses
  Classes, Controls, Grids;

type
  TStringGrid1 = class(TStringGrid)
  protected
    procedure MouseDown(Button: TMouseButton; Shift: TShiftState;
      X, Y: Integer); override;
  end;

procedure Register;

implementation

procedure Register;
begin
  RegisterComponents('Samples', [TStringGrid1]);
end;

procedure TStringGrid1.MouseDown(Button: TMouseButton; Shift: TShiftState;
  X, Y: Integer);
begin
  if Button = mbRight then
    Button := mbLeft;
  inherited MouseDown(Button, Shift, X, Y);
end;

end.