Two related changes that both affect how cursor positions might change on single line edits.

dart/site/try/src/interaction_manager.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library trydart.interaction_manager;
 
 import 'dart:html';
 
 import 'dart:convert' show
     JSON;
@@ skipping 302 matching lines @@
 
     if (normalizedNodes.length == 1) {
       Node node = normalizedNodes.single;
       if (node is Element && node.classes.contains('lineNumber')) {
         print('Single line change: ${node.outerHtml}');
 
         removeShadowRootPolyfill(node);
 
         String currentText = node.text;
 
-        trySelection = new TrySelection(node, selection);
+        trySelection = trySelection.copyWithRoot(node);
         trySelection.updateText(currentText);
 
         editor.isMalformedInput = false;
         int offset = 0;
         List<Node> nodes = <Node>[];
 
         String state = '';
         Element previousLine = node.previousElementSibling;
         if (previousLine != null) {
           state = previousLine.getAttribute('dart-state');
@@ skipping 579 matching lines @@
       token.start == '"""' ||
       token.start == "r'''" ||
       token.start == 'r"""';
 }
 
 void normalizeMutationRecord(MutationRecord record,
                              TrySelection selection,
                              Set<Node> normalizedNodes) {
   for (Node node in record.addedNodes) {
     if (node.parent == null) continue;
+    normalizedNodes.add(findLine(node));
+    if (node is Text) continue;
     StringBuffer buffer = new StringBuffer();
     int selectionOffset = htmlToText(node, buffer, selection);
     Text newNode = new Text('$buffer');
     node.replaceWith(newNode);
-    normalizedNodes.add(findLine(newNode));
     if (selectionOffset != -1) {
       selection.anchorNode = newNode;
       selection.anchorOffset = selectionOffset;
     }
   }
   if (!record.removedNodes.isEmpty) {
     normalizedNodes.add(findLine(record.target));
   }
   if (record.type == "characterData") {
     normalizedNodes.add(findLine(record.target));
@@ skipping 30 matching lines @@
   return text.split(new RegExp('^', multiLine: true));
 }
 
 void removeCodeCompletion() {
   List<Node> highlighting =
       mainEditorPane.querySelectorAll('.dart-code-completion');
   for (Element element in highlighting) {
     element.remove();
   }
 }