Ensure event helper is installed when using a JPopupMenu.

core/src/main/java/com/github/weisj/darklaf/ui/popupmenu/DarkPopupMenuUI.java

@@ -56,6 +56,10 @@ public class DarkPopupMenuUI extends BasicPopupMenuUI {
         return new DarkPopupMenuUI();
     }
 
+    public DarkPopupMenuUI() {
+        EventHelperUtil.installEventHelper();
+    }
+
     @Override
     public void paint(final Graphics g, final JComponent c) {
         Window window = SwingUtilities.getWindowAncestor(c);

core/src/main/java/com/github/weisj/darklaf/ui/popupmenu/EventHelperUtil.java

@@ -0,0 +1,66 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2020 Jannis Weis
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ */
+package com.github.weisj.darklaf.ui.popupmenu;
+
+import javax.swing.plaf.basic.BasicLookAndFeel;
+
+public class EventHelperUtil {
+
+    private static boolean eventHelperInstallerFlagSet;
+
+    public static void installEventHelper() {
+        if (eventHelperInstallerFlagSet) return;
+        eventHelperInstallerFlagSet = true;
+        new DummyBasicLookAndFeel().initialize();
+    }
+
+    private static class DummyBasicLookAndFeel extends BasicLookAndFeel {
+
+        @Override
+        public String getName() {
+            return null;
+        }
+
+        @Override
+        public String getID() {
+            return null;
+        }
+
+        @Override
+        public String getDescription() {
+            return null;
+        }
+
+        @Override
+        public boolean isNativeLookAndFeel() {
+            return false;
+        }
+
+        @Override
+        public boolean isSupportedLookAndFeel() {
+            return false;
+        }
+    }
+}

core/src/main/java/com/github/weisj/darklaf/ui/popupmenu/MouseGrabberUtil.java

@@ -37,9 +37,7 @@ import com.github.weisj.darklaf.util.DarkUIUtil;
 
 public class MouseGrabberUtil {
 
-    private MouseGrabberUtil() {
-
-    }
+    private MouseGrabberUtil() {}
 
     private static MouseGrabber mouseGrabber;
 
@@ -58,7 +56,7 @@ public class MouseGrabberUtil {
         }
     }
 
-    public static ChangeListener getOldMouseGrabber() {
+    private static ChangeListener getOldMouseGrabber() {
         MenuSelectionManager menuSelectionManager = MenuSelectionManager.defaultManager();
         for (ChangeListener listener : menuSelectionManager.getChangeListeners()) {
             if (listener == null) continue;
@@ -78,7 +76,7 @@ public class MouseGrabberUtil {
      * This Method is responsible for removing the old MouseGrabber from the AppContext, to be able to add our own
      * implementation for it that is a bit more generous with closing the popup.
      */
-    public static void uninstallOldMouseGrabber(final ChangeListener oldMouseGrabber) {
+    private static void uninstallOldMouseGrabber(final ChangeListener oldMouseGrabber) {
         if (oldMouseGrabber == null) return;
         MenuSelectionManager menuSelectionManager = MenuSelectionManager.defaultManager();
         menuSelectionManager.removeChangeListener(oldMouseGrabber);