Chart: position plugins using Panes instead of Groups

It seems that groups are not well suited to positioning nodes on top of
the graph. Replacing the Groups for the Plugins by Panes fixes an issue
where the zoom rectangle would shift to the upper left corner of the
chart area whenever there was a relayout while a rectangle zoom was
ongoing.

chartfx-chart/src/main/java/io/fair_acc/chartfx/Chart.java

@@ -27,7 +27,6 @@
 import javafx.css.*;
 import javafx.geometry.*;
 import javafx.scene.CacheHint;
-import javafx.scene.Group;
 import javafx.scene.Node;
 import javafx.scene.canvas.Canvas;
 import javafx.scene.control.Control;
@@ -99,17 +98,16 @@ public abstract class Chart extends Region implements EventSource {
     protected final ChartLayoutAnimator animator = new ChartLayoutAnimator(this);
 
     protected final ObservableList<Axis> axesList = FXCollections.observableList(new NoDuplicatesList<>());
-    private final Map<ChartPlugin, Group> pluginGroups = new HashMap<>();
+    private final Map<ChartPlugin, Pane> pluginPanes = new HashMap<>();
     private final ObservableList<ChartPlugin> plugins = FXCollections.observableList(new LinkedList<>());
     private final ObservableList<DataSet> datasets = FXCollections.observableArrayList();
     protected final ObservableList<DataSet> allDataSets = FXCollections.observableArrayList();
     private final ObservableList<Renderer> renderers = FXCollections.observableArrayList();
 
-
     // Inner canvas for the drawn content
     protected final ResizableCanvas canvas = StyleUtil.addStyles(new ResizableCanvas(), "chart-canvas");
-    protected final Pane canvasForeground = new Pane();
-    protected final Group pluginsArea = Chart.createChildGroup();
+    protected final Pane canvasForeground = FXUtils.createUnmanagedPane();
+    protected final Pane pluginsArea = FXUtils.createUnmanagedPane();
 
     // Area where plots get drawn
     protected final Pane plotBackground = StyleUtil.addStyles(new Pane(), "chart-plot-background");
@@ -814,20 +812,20 @@ protected void layoutPluginsChildren() {
 
     protected void pluginAdded(final ChartPlugin plugin) {
         plugin.setChart(Chart.this);
-        final Group group = Chart.createChildGroup();
-        Bindings.bindContent(group.getChildren(), plugin.getChartChildren());
-        pluginGroups.put(plugin, group);
+        final Pane pane = FXUtils.createUnmanagedPane();
+        Bindings.bindContent(pane.getChildren(), plugin.getChartChildren());
+        pluginPanes.put(plugin, pane);
     }
 
     // -------------- STYLESHEET HANDLING
     // ------------------------------------------------------------------------------
 
     protected void pluginRemoved(final ChartPlugin plugin) {
         plugin.setChart(null);
-        final Group group = pluginGroups.remove(plugin);
-        Bindings.unbindContent(group, plugin.getChartChildren());
-        group.getChildren().clear();
-        pluginsArea.getChildren().remove(group);
+        final Pane pane = pluginPanes.remove(plugin);
+        Bindings.unbindContent(pane, plugin.getChartChildren());
+        pane.getChildren().clear();
+        pluginsArea.getChildren().remove(pane);
     }
 
     protected void pluginsChanged(final ListChangeListener.Change<? extends ChartPlugin> change) {
@@ -905,7 +903,7 @@ protected void updateLegend(final List<DataSet> dataSets, final List<Renderer> r
     }
 
     protected void updatePluginsArea() {
-        var pluginChildren = plugins.stream().map(pluginGroups::get).collect(Collectors.toList());
+        var pluginChildren = plugins.stream().map(pluginPanes::get).collect(Collectors.toList());
         pluginsArea.getChildren().setAll(pluginChildren);
         fireInvalidated(ChartBits.ChartPlugins);
     }
@@ -927,13 +925,4 @@ private void ensureJavaFxPulse() {
     public static List<CssMetaData<? extends Styleable, ?>> getClassCssMetaData() {
         return CSS.getCssMetaData();
     }
-
-    protected static Group createChildGroup() {
-        final Group group = new Group();
-        group.setManaged(false);
-        group.setAutoSizeChildren(false);
-        group.relocate(0, 0);
-        return group;
-    }
-
 }

chartfx-chart/src/main/java/io/fair_acc/chartfx/utils/FXUtils.java

@@ -21,6 +21,7 @@
 import javafx.scene.Parent;
 import javafx.scene.Scene;
 
+import javafx.scene.layout.Pane;
 import javafx.stage.Window;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -390,4 +391,10 @@ public static void registerLayoutHooks(Node node, Runnable preLayoutAction, Runn
         });
     }
 
+    public static Pane createUnmanagedPane() {
+        final Pane pane = new Pane();
+        pane.setManaged(false);
+        pane.relocate(0, 0);
+        return pane;
+    }
 }