The Tor Browser / file revision

 // frame.onStep can coexist with breakpoints.

 var g = newGlobal();

 var dbg = Debugger(g);

 var log = '';

 dbg.onEnterFrame = function (frame) {

     var handler = {hit: function () { log += 'B'; }};

     var lines = frame.script.getAllOffsets();

     for (var line in lines) {

         line = Number(line);

         var offs = lines[line];

         for (var i = 0; i < offs.length; i++)

             frame.script.setBreakpoint(offs[i], handler);

     }

     frame.onStep = function () { log += 's'; };

 };

 g.eval("one = 1;\n" +

        "two = 2;\n" +

        "three = 3;\n" +

        "four = 4;\n");

 assertEq(g.four, 4);

 // Breakpoints hit on all four lines.

 assertEq(log.replace(/[^B]/g, ''), 'BBBB');

 // onStep was called between each pair of breakpoints.

 assertEq(/BB/.exec(log), null);