Fix firefox bug and Issue #1

* Fix Bug in Firefox when dragging over nested elements.
* Fix Issue #1: overClass (.dnd-over) stays after drag ended

Author: marcojakob

example/html5_dnd_example.dart

@@ -12,8 +12,8 @@ part 'codeblocks.dart';
 
 main() {
   // Uncomment to enable logging.
-//  Logger.root.onRecord.listen(new PrintHandler().call);
-//  Logger.root.level = Level.FINEST;
+  Logger.root.onRecord.listen(new PrintHandler().call);
+  Logger.root.level = Level.FINEST;
 
   // Drag and Drop
   sectionDraggableAndDropzone();

lib/draggable.dart

@@ -40,7 +40,7 @@ class DraggableGroup extends Group {
    * returns 'text' as type with an empty String as data.
    */
   DragDataFunction dragDataFunction = (Element draggable) {
-    return {'text': ''};
+    return {'Text': ' '};
   };
 
   /**
@@ -400,10 +400,11 @@ class DragImage {
       // Make sure that mouse events are forwarded to the layer below.
       if (html5.supportsPointerEvents) {
         _polyfill = image;
-        _polyfill.style.pointerEvents = 'none';
       } else {
+        // IE9 and IE10 support pointer-events on SVGs only.
         _polyfill = _createSvgElement();
       }
+      _polyfill.style.pointerEvents = 'none';
 
       // Add some transparency.
       _polyfill.style.opacity = polyfillOpacity;
@@ -429,8 +430,6 @@ class DragImage {
           height="${image.height}" 
           />
         </svg>
-    """)
-    // Event IE9 and IE10 support pointer-events on SVGs.
-    ..style.pointerEvents = 'none';
+    """);
   }
 }

lib/dropzone.dart

@@ -96,21 +96,31 @@ class DropzoneGroup extends Group {
         return; // Return here as drop is not accepted.
       }
 
-      _logger.finest('onDragEnter {dragOverElements.length before adding: ${currentDragOverElements.length}}');
-      currentDragOverElements.add(mouseEvent.target);
-      
       // Only handle dropzone element itself and not any of its children.
-      if (currentDragOverElements.length == 1) {
+      if (currentDragOverElements.isEmpty) {
         _logger.finest('firing onDragEnter');
         if (currentDraggableGroup.overClass != null) {
-          element.classes.add(currentDraggableGroup.overClass);
+          String overClass = currentDraggableGroup.overClass;
+          element.classes.add(overClass);
+          
+          // Make sure overClass is removed when drag ended. Is necessary 
+          // because if drag is aborted (e.g. with esc-key), no dragLeave or
+          // drop event is fired on the dropzone.
+          StreamSubscription dragEndSub;
+          dragEndSub = currentDraggable.onDragEnd.listen((_) {
+            element.classes.remove(overClass);
+            dragEndSub.cancel();
+          });
         }
 
         if (_onDragEnter != null) {
           _onDragEnter.add(new DropzoneEvent(currentDraggable, 
               element, mouseEvent));
         }
       }
+      
+      _logger.finest('onDragEnter {dragOverElements.length before adding: ${currentDragOverElements.length}}');
+      currentDragOverElements.add(mouseEvent.target);
     }));
 
     // Drag Over.
@@ -141,8 +151,8 @@ class DropzoneGroup extends Group {
 
       // Firefox fires too many onDragLeave events. This condition fixes it. 
       if (mouseEvent.target != mouseEvent.relatedTarget) {
-        _logger.finest('onDragLeave {dragOverElements.length before removing: ${currentDragOverElements.length}}');
         currentDragOverElements.remove(mouseEvent.target);
+        _logger.finest('onDragLeave {dragOverElements.length after removing: ${currentDragOverElements.length}}');
       }
 
       // Only handle on dropzone element and not on any of its children.
@@ -167,10 +177,6 @@ class DropzoneGroup extends Group {
       // Stops browsers from redirecting.
       mouseEvent.preventDefault(); 
 
-      if (currentDraggableGroup.overClass != null) {
-        element.classes.remove(currentDraggableGroup.overClass);
-      }
-      
       if (_onDrop != null) {
         _onDrop.add(new DropzoneEvent(currentDraggable, element, 
             mouseEvent));

lib/html5_dnd.dart

@@ -33,6 +33,9 @@ DraggableGroup currentDraggableGroup;
 /// was fired on the real [dropzoneElement] and not on its children.
 Set<Element> currentDragOverElements = new Set<Element>();
 
+/**
+ * Abstract superclass for all groups containing drag and drop elements.
+ */
 abstract class Group {
   /// Map of all installed elements inside this group with their subscriptions.
   Map<Element, List<StreamSubscription>> installedElements = 
@@ -70,5 +73,4 @@ abstract class Group {
   void uninstallAll(List<Element> elements) {
     elements.forEach((Element e) => uninstall(e));
   }
-  
 }

lib/sortable.dart

@@ -76,8 +76,13 @@ class SortableGroup extends DraggableGroup {
    */
   SortableGroup({String handle}) :
     _dropzoneGroup = new DropzoneGroup(),
-    super(handle: handle);
-    
+    super(handle: handle) {
+    // Disable overClass by default for sortable as we're usually only over the 
+    // placeholder and not over a dropzone. Same for draggingClass as the 
+    // dragged element is replaced by the placeholder and thus not visible.
+    overClass = null;
+    draggingClass = null;
+  }
 
   /**
    * Installs sortable behaviour on [element] and registers it in this group.