CodeMirror 2: Breakpoint demo
CodeMirror.fromTextArea(document.getElementById("code"), { lineNumbers: true, onGutterClick: function(cm, n) { var info = cm.lineInfo(n); if (info.markerText) cm.clearMarker(n); else cm.setMarker(n, "<span style=\"color: #900\">●</span> %N%"); } });
Click the line-number gutter to add or remove 'breakpoints'.