Fix issues that broke editing on browsers without Shadow DOM support.

dart/site/try/src/interaction_manager.dart

Index: dart/site/try/src/interaction_manager.dart
diff --git a/dart/site/try/src/interaction_manager.dart b/dart/site/try/src/interaction_manager.dart
index ecd7663b1ab51ba60c2eb35568dcb34f33c394c9..97c06d460eef977fa048c39183e68c80448643bf 100644
--- a/dart/site/try/src/interaction_manager.dart
+++ b/dart/site/try/src/interaction_manager.dart
@@ -236,6 +236,7 @@ class InteractionContext extends InteractionManager {
   void onKeyUp(KeyboardEvent event) => state.onKeyUp(event);
 
   void onMutation(List<MutationRecord> mutations, MutationObserver observer) {
+    workAroundFirefoxBug();
     try {
       try {
         return state.onMutation(mutations, observer);
@@ -308,7 +309,6 @@ abstract class InteractionState implements InteractionManager {
   void set state(InteractionState newState);
 
   void onStateChanged(InteractionState previous) {
-    print('State change ${previous.runtimeType} -> ${runtimeType}.');
   }
 
   void transitionToInitialState() {
@@ -336,10 +336,8 @@ class InitialState extends InteractionState {
 
   void onKeyUp(KeyboardEvent event) {
     if (computeHasModifier(event)) {
-      print('onKeyUp (modified)');
       onModifiedKeyUp(event);
     } else {
-      print('onKeyUp (unmodified)');
       onUnmodifiedKeyUp(event);
     }
   }
@@ -413,8 +411,6 @@ class InitialState extends InteractionState {
   }
 
   void onMutation(List<MutationRecord> mutations, MutationObserver observer) {
-    print('onMutation');
-
     removeCodeCompletion();
 
     Selection selection = window.getSelection();
@@ -456,7 +452,11 @@ class InitialState extends InteractionState {
 
         node.parent.insertAllBefore(nodes, node);
         node.remove();
-        trySelection.adjust(selection);
+        if (mainEditorPane.contains(trySelection.anchorNode)) {
+          // Sometimes the anchor node is removed by the above call. This has
+          // only been observed in Firefox, and is hard to reproduce.
+          trySelection.adjust(selection);
+        }
 
         // TODO(ahe): We know almost exactly what has changed.  It could be
         // more efficient to only communicate what changed.
@@ -1206,7 +1206,10 @@ void normalizeMutationRecord(MutationRecord record,
     }
     normalizedNodes.add(line);
   }
-  if (record.type == "characterData") {
+  if (record.type == "characterData" && record.target.parent != null) {
+    // At least Firefox sends a "characterData" record whose target is the
+    // deleted text node. It also sends a record where "removedNodes" isn't
+    // empty whose target is the parent (which we are interested in).
     normalizedNodes.add(findLine(record.target));
   }
 }
@@ -1259,3 +1262,23 @@ bool isCompilerStageMarker(String message) {
       message == "Compiling..." ||
       message.startsWith('Compiled ');
 }
+
+void workAroundFirefoxBug() {
+  Selection selection = window.getSelection();
+  if (!isCollapsed(selection)) return;
+  Node node = selection.anchorNode;
+  int offset = selection.anchorOffset;
+  if (selection.anchorNode is Element && selection.anchorOffset != 0) {
+    // In some cases, Firefox reports the wrong anchorOffset (always seems to
+    // be 6) when anchorNode is an Element. Moving the cursor back and forth
+    // adjusts the anchorOffset.
+    // Safari can also reach this code, but the offset isn't wrong, just
+    // inconsistent.  After moving the cursor back and forth, Safari will make
+    // the offset relative to a text node.
+    selection
+        ..modify('move', 'backward', 'character')
+        ..modify('move', 'forward', 'character');
+    print('Selection adjusted $node@$offset -> '
+          '${selection.anchorNode}@${selection.anchorOffset}.');
+  }
+}