8280994: [XWayland] Drag and Drop does not work in java -> wayland app direction

src/java.desktop/share/classes/sun/awt/SunToolkit.java

@@ -1883,6 +1883,11 @@ public boolean isNativeGTKAvailable() {
         return false;
     }
 
+    /**
+     * Checks if the system is running Linux with the Wayland server.
+     *
+     * @return true if running on Wayland, false otherwise
+     */
     public boolean isRunningOnWayland() {
         return false;
     }

src/java.desktop/unix/classes/sun/awt/X11/XDragSourceContextPeer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2003, 2015, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2023, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -27,6 +27,7 @@
 
 import java.awt.Component;
 import java.awt.Cursor;
+import java.awt.Toolkit;
 import java.awt.Window;
 
 import java.awt.datatransfer.DataFlavor;
@@ -392,6 +393,40 @@ private boolean updateSourceAction(int state) {
         return true;
     }
 
+    /**
+     * Our X11 code expects the drop target window to be a top level window
+     * and to have the XA_WM_STATE property.
+     * This is not true when performing drag and drop from XWayland
+     * to a native Wayland application.
+     * In this case XWayland creates a dummy window with only one property,
+     * XdndAware.
+     *
+     * @param window to test
+     * @return true if window has XdndAware property when running under Wayland
+     */
+    private static boolean isXWaylandDndAwareWindow(long window) {
+        Toolkit toolkit = Toolkit.getDefaultToolkit();
+        if (!(toolkit instanceof SunToolkit)
+                || !((SunToolkit) toolkit).isRunningOnWayland()) {
+            return false;
+        }
+
+        WindowPropertyGetter wpg =
+            new WindowPropertyGetter(window, XDnDConstants.XA_XdndAware, 0, 1,
+                                     false, XConstants.AnyPropertyType);
+
+        try {
+            int status =
+                wpg.execute(XErrorHandler.IgnoreBadWindowHandler.getInstance());
+
+            return status == XConstants.Success
+                   && wpg.getData() != 0
+                   && wpg.getActualType() == XAtom.XA_ATOM;
+        } finally {
+            wpg.dispose();
+        }
+    }
+
     /**
      * Returns the client window under the specified root subwindow.
      */
@@ -400,6 +435,10 @@ private static long findClientWindow(long window) {
             return window;
         }
 
+        if (isXWaylandDndAwareWindow(window)) {
+            return window;
+        }
+
         Set<Long> children = XlibUtil.getChildWindows(window);
         for (Long child : children) {
             long win = findClientWindow(child);