Debugging the the script

Use OnRunLine. For example:

procedure TForm1.OnRunLine(Sender: TfsScript; const UnitName, SourcePos: String);
var
  pt: TPoint;
begin
// locate the unit with UnitName name
  ...
// locate the line with pt.Y number
  pt := fsPosToPoint(SourcePos);

  FStopped := True;
  while FStopped do
    Application.ProcessMessages;
end;

Examine the demo located in the DEMOS\Main folder.