ADDITIONAL SYSTEM INFORMATION :
Windows x64

A DESCRIPTION OF THE PROBLEM :
Dragging files from Windows Explorer to a Java application hangs. Not each time but very often.
I have implemented a DroppableDesktopPane (see source code) and dropping a file on that DesktopPane sometimes hangs (very often).
This issue is new with JDK 15.0.2 and JDK 11.0.10. It is working fine with the previous release JDK 15.0.1 and JDK 11.0.9.

REGRESSION : Last worked in version 15.0.1, 11.0.9

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Drag a file on the DroppableDesktopPane.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No hanging
ACTUAL -
Application hangs. Stack trace:
UPSEClient15 [Java Application]	
	StartUpFrame at localhost:64016 (Suspended)	
		Thread [Timer-0] (Suspended)	
			waiting for: TaskQueue  (id=40)	
			Object.wait(long) line: not available [native method]	
			TaskQueue(Object).wait() line: 321	
			TimerThread.mainLoop() line: 527	
			TimerThread.run() line: 506	
		Thread [AWT-EventQueue-0] (Suspended)	
			WInputMethod.openCandidateWindow(WComponentPeer, int, int) line: not available [native method]	
			WInputMethod$1.run() line: 622	
			InvocationEvent.dispatch() line: 316	
			EventQueue.dispatchEventImpl(AWTEvent, Object) line: 770	
			EventQueue$4.run() line: 721	
			EventQueue$4.run() line: 715	
			AccessController.executePrivileged(PrivilegedAction<T>, AccessControlContext, Class<?>) line: 753	
			AccessController.doPrivileged(PrivilegedAction<T>, AccessControlContext) line: 391	
			ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(PrivilegedAction<T>, AccessControlContext, AccessControlContext) line: 85	
			ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(PrivilegedAction<T>, AccessControlContext) line: 95	
			EventQueue$5.run() line: 745	
			EventQueue$5.run() line: 743	
			AccessController.executePrivileged(PrivilegedAction<T>, AccessControlContext, Class<?>) line: 753	
			AccessController.doPrivileged(PrivilegedAction<T>, AccessControlContext) line: 391	
			ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(PrivilegedAction<T>, AccessControlContext, AccessControlContext) line: 85	
			EventQueue.dispatchEvent(AWTEvent) line: 742	
			EventDispatchThread.pumpOneEventForFilters(int) line: 203	
			EventDispatchThread.pumpEventsForFilter(int, Conditional, EventFilter) line: 124	
			EventDispatchThread.pumpEventsForHierarchy(int, Conditional, Component) line: 113	
			EventDispatchThread.pumpEvents(int, Conditional) line: 109	
			EventDispatchThread.pumpEvents(Conditional) line: 101	
			EventDispatchThread.run() line: 90	
		Thread [DestroyJavaVM] (Suspended)	
		Daemon Thread [mysql-cj-abandoned-connection-cleanup] (Suspended)	
			waiting for: ReferenceQueue$Lock  (id=38)	
			Object.wait(long) line: not available [native method]	
			ReferenceQueue<T>.remove(long) line: 155	
			AbandonedConnectionCleanupThread.run() line: 91	
			ThreadPoolExecutor.runWorker(ThreadPoolExecutor$Worker) line: 1130	
			ThreadPoolExecutor$Worker.run() line: 630	
			Thread.run() line: 832	
		Thread [Thread-2] (Suspended)	
			owns: BufferedInputStream  (id=39)	
			SocketDispatcher.read0(FileDescriptor, long, int) line: not available [native method]	
			SocketDispatcher.read(FileDescriptor, long, int) line: 46	
			NioSocketImpl.tryRead(FileDescriptor, byte[], int, int) line: 261	
			NioSocketImpl.implRead(byte[], int, int) line: 312	
			NioSocketImpl.read(byte[], int, int) line: 350	
			NioSocketImpl$1.read(byte[], int, int) line: 803	
			Socket$SocketInputStream.read(byte[], int, int) line: 981	
			BufferedInputStream.fill() line: 244	
			BufferedInputStream.read() line: 263	
			DataInputStream.readInt() line: 391	
			TCPThread.run() line: 311	
		Daemon System Thread [Reference Handler] (Suspended)	
			Reference<T>.waitForReferencePendingList() line: not available [native method]	
			Reference<T>.processPendingReferences() line: 241	
			Reference$ReferenceHandler.run() line: 213	
		Daemon System Thread [Finalizer] (Suspended)	
			waiting for: ReferenceQueue$Lock  (id=41)	
			Object.wait(long) line: not available [native method]	
			ReferenceQueue<T>.remove(long) line: 155	
			ReferenceQueue<T>.remove() line: 176	
			Finalizer$FinalizerThread.run() line: 170	
		Daemon System Thread [Signal Dispatcher] (Suspended)	
		Daemon System Thread [Attach Listener] (Suspended)	
		Daemon System Thread [Common-Cleaner] (Suspended)	
			waiting for: ReferenceQueue$Lock  (id=42)	
			Object.wait(long) line: not available [native method]	
			ReferenceQueue<T>.remove(long) line: 155	
			CleanerImpl.run() line: 148	
			InnocuousThread(Thread).run() line: 832	
			InnocuousThread.run() line: 134	
		Daemon System Thread [Notification Thread] (Suspended)	
		Daemon System Thread [Java2D Disposer] (Suspended)	
			waiting for: ReferenceQueue$Lock  (id=43)	
			Object.wait(long) line: not available [native method]	
			ReferenceQueue<T>.remove(long) line: 155	
			ReferenceQueue<T>.remove() line: 176	
			Disposer.run() line: 144	
			Thread.run() line: 832	
		System Thread [AWT-Shutdown] (Suspended)	
			waiting for: Object  (id=44)	
			Object.wait(long) line: not available [native method]	
			Object.wait() line: 321	
			AWTAutoShutdown.run() line: 291	
			Thread.run() line: 832	
		Daemon System Thread [AWT-Windows] (Suspended)	
			WToolkitThreadBlockedHandler.startSecondaryEventLoop() line: not available [native method]	
			WToolkitThreadBlockedHandler.enter() line: 508	
			WDropTargetContextPeer(SunDropTargetContextPeer).postDropTargetEvent(Component, int, int, int, int, long[], long, int, boolean) line: 593	
			WDropTargetContextPeer(SunDropTargetContextPeer).handleEnterMessage(Component, int, int, int, int, long[], long) line: 302	
			WToolkit.eventLoop() line: not available [native method]	
			WToolkit.run() line: 361	
			Thread.run() line: 832	
		Daemon System Thread [TimerQueue] (Suspended)	
			Unsafe.park(boolean, long) line: not available [native method]	
			LockSupport.park() line: 341	
			AbstractQueuedSynchronizer$ConditionNode.block() line: 505	
			ForkJoinPool.managedBlock(ForkJoinPool$ManagedBlocker) line: 3137	
			AbstractQueuedSynchronizer$ConditionObject.await() line: 1614	
			DelayQueue<E>.take() line: 217	
			TimerQueue.run() line: 171	
			Thread.run() line: 832	
	C:\Program Files\Java\jdk-15.0.2\bin\javaw.exe (23 Feb 2021, 06:27:09)	


---------- BEGIN SOURCE ----------
/*
		A JDesktopPane which support Drag&Drop.
*/

import java.io.*;
import java.awt.dnd.*;
import java.awt.datatransfer.*;
import java.util.*;


public class DroppableDesktopPane extends javax.swing.JDesktopPane implements DropTargetListener
{
	private static final long serialVersionUID = 1L;
	
	DropTarget  dropTarget;
    MainFrame   mFrame;

    public DroppableDesktopPane(MainFrame mf)
    {
        super();
        mFrame = mf;
        dropTarget = new DropTarget(this, this);
    }


    public void dragEnter(DropTargetDragEvent e)
	{
	    e.acceptDrag(DnDConstants.ACTION_COPY);
	}

	public void dragExit(DropTargetEvent e)
	{

	}

	public void dragOver(DropTargetDragEvent e)
	{
	    e.acceptDrag(DnDConstants.ACTION_COPY);
	}

	public void dropActionChanged(DropTargetDragEvent e)
	{
	    e.acceptDrag(DnDConstants.ACTION_COPY);
	}

	public synchronized void drop(DropTargetDropEvent e)
	{
	    try
	    {
			DataFlavor uriListFlavor = new DataFlavor("text/uri-list;class=java.lang.String");
	        Transferable tr = e.getTransferable();

	        if (tr.isDataFlavorSupported(DataFlavor.javaFileListFlavor))
	        {
	            e.acceptDrop(DnDConstants.ACTION_COPY);
	            java.util.List<?> fileList = (java.util.List<?>)tr.getTransferData(DataFlavor.javaFileListFlavor);

	            Iterator<?> it = fileList.iterator();
	            while (it.hasNext())
	            {
	                File file = (File)it.next();
	                if (file != null)
	                    mFrame.doOpen(file);
	            }
	            e.getDropTargetContext().dropComplete(true);
	        }
			else if (tr.isDataFlavorSupported(uriListFlavor))  // Linux java bug 4899516: drop doesn't give a list of file
			{
				e.acceptDrop(DnDConstants.ACTION_COPY);
				String data = (String)tr.getTransferData(uriListFlavor);
				java.util.List<?> fileList = textURIListToFileList(data);

				Iterator<?> it = fileList.iterator();
				while (it.hasNext())
				{
					File file = (File)it.next();
					// open the file
				}
				e.getDropTargetContext().dropComplete(true);
			}
	        else
	            e.rejectDrop();
	    }
	    catch (IOException exio)
	    {
	        e.rejectDrop();
	    }
	    catch (UnsupportedFlavorException ex)
	    {
	        e.rejectDrop();
	    }
		catch (ClassNotFoundException ex2)
		{
			e.rejectDrop();
		}
	}


	private static java.util.List<File> textURIListToFileList(String data)
	{
		java.util.List<File> list = new java.util.ArrayList<File>(1);
		for (java.util.StringTokenizer st = new java.util.StringTokenizer(data, "\r\n"); st.hasMoreTokens();)
		{
			String s = st.nextToken();
			if (s.startsWith("#"))
			{
				// the line is a comment (as per the RFC 2483)
				continue;
			}
			try
			{
				java.net.URI uri = new java.net.URI(s);
				java.io.File file = new java.io.File(uri);
				list.add(file);
			} catch (java.net.URISyntaxException e)
			{
				// malformed URI
			} catch (IllegalArgumentException e)
			{
				// the URI is not a valid 'file:' URI
			}
		}
		return list;
	}

}

---------- END SOURCE ----------

FREQUENCY : often