Java 9 StageHelper Replacement

The ability to grab all stages was made private in Java 9 so I have to
improvise. My workaround for now is to just maintain a non-threadsafe
package-private statically-initialized list of all DockPanes to use for
scene graph traversal. This works because every Node, which DockPane is
a descendant of, knows its scene which in turn is aware of its stage.

src/main/java/org/dockfx/DockEvent.java

@@ -30,7 +30,7 @@
 import javafx.scene.input.PickResult;
 
 /**
- * Base class for DockFX events. Each DockFX event has associated an event source, event target and
+ * Base class for DockFX events. Each DockFX event has an associated event source, event target and
  * an event type. The event source specifies for an event handler the object on which that handler
  * has been registered and which sent the event to it. The event target defines the path through
  * which the event will travel when posted. The event type provides additional classification to

src/main/java/org/dockfx/DockPane.java

@@ -20,6 +20,8 @@
 
 package org.dockfx;
 
+import java.util.ArrayList;
+import java.util.List;
 import java.util.Stack;
 
 import com.sun.javafx.css.StyleManager;
@@ -53,6 +55,11 @@
  * @since DockFX 0.1
  */
 public class DockPane extends StackPane implements EventHandler<DockEvent> {
+  /**
+   * Package-private internal list of all DockPanes for event mouse picking.
+   */
+  static List<DockPane> dockPanes = new ArrayList<DockPane>();
+
   /**
    * The current root node of this dock pane's layout.
    */
@@ -184,6 +191,7 @@ public final boolean isDockRoot() {
    */
   public DockPane() {
     super();
+    DockPane.dockPanes.add(this);
 
     this.addEventHandler(DockEvent.ANY, this);
     this.addEventFilter(DockEvent.ANY, new EventHandler<DockEvent>() {

src/main/java/org/dockfx/DockTitleBar.java

@@ -21,6 +21,7 @@
 package org.dockfx;
 
 import java.util.HashMap;
+import java.util.List;
 import java.util.Stack;
 
 import com.sun.javafx.stage.StageHelper;
@@ -230,10 +231,14 @@ private void pickEventTarget(Point2D location, EventTask eventTask, Event explic
     // RFE for public scene graph traversal API filed but closed:
     // https://bugs.openjdk.java.net/browse/JDK-8133331
 
-    ObservableList<Stage> stages =
-        FXCollections.unmodifiableObservableList(StageHelper.getStages());
+    List<DockPane> dockPanes = DockPane.dockPanes;
+
     // fire the dock over event for the active stages
-    for (Stage targetStage : stages) {
+    for (DockPane dockPane : dockPanes) {
+      Window window = dockPane.getScene().getWindow();
+      if (!(window instanceof Stage)) continue;
+      Stage targetStage = (Stage) window;
+
       // obviously this title bar does not need to receive its own events
       // though users of this library may want to know when their
       // dock node is being dragged by subclassing it or attaching