AssertionError at javafx.embed.swing.SwingDnD.getDragSource

If you have two WebViews embedded in separate JFXPanels, attempting to drag an image from one WebView to another results in this exception being thrown infinitely when assertions are on:

java.lang.AssertionError
at javafx.embed.swing.SwingDnD.getDragSource(SwingDnD.java:324)
at javafx.embed.swing.SwingDnD.access$500(SwingDnD.java:71)
at javafx.embed.swing.SwingDnD$DnDTransferable.getTransferData(SwingDnD.java:439)
at sun.awt.datatransfer.DataTransferer.translateTransferable(DataTransferer.java:1180)
at sun.awt.datatransfer.DataTransferer$6.run(DataTransferer.java:2304)
at sun.awt.datatransfer.DataTransferer.processDataConversionRequests(DataTransferer.java:2361)
at sun.awt.X11.XSelection.waitForSelectionNotify(XSelection.java:179)
at sun.awt.X11.XSelection.getData(XSelection.java:295)
at sun.awt.X11.XDnDDropTargetProtocol.getData(XDnDDropTargetProtocol.java:859)
at sun.awt.X11.XDropTargetContextPeer.getNativeData(XDropTargetContextPeer.java:151)
at sun.awt.dnd.SunDropTargetContextPeer.getTransferData(SunDropTargetContextPeer.java:263)
at sun.awt.datatransfer.TransferableProxy.getTransferData(TransferableProxy.java:73)
at java.awt.dnd.DropTargetContext$TransferableProxy.getTransferData(DropTargetContext.java:376)
at javafx.embed.swing.DataFlavorUtils.readAllData(DataFlavorUtils.java:148)
at javafx.embed.swing.SwingDragSource.updateData(SwingDragSource.java:91)
at javafx.embed.swing.SwingDragSource.updateContents(SwingDragSource.java:34)
at javafx.embed.swing.SwingDragSource.setContents(SwingDragSource.java:41)
at javafx.embed.swing.SwingDragSource.<init>(SwingDragSource.java:29)
at javafx.embed.swing.SwingDnD$3.dragEnter(SwingDnD.java:146)
at java.awt.dnd.DropTarget.dragEnter(DropTarget.java:357)
at sun.awt.dnd.SunDropTargetContextPeer.processEnterMessage(SunDropTargetContextPeer.java:330)
at sun.awt.X11.XDropTargetContextPeer.processEnterMessage(XDropTargetContextPeer.java:165)
at sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchEnterEvent(SunDropTargetContextPeer.java:798)
at sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchEvent(SunDropTargetContextPeer.java:766)
at sun.awt.dnd.SunDropTargetEvent.dispatch(SunDropTargetEvent.java:48)
at java.awt.Component.dispatchEventImpl(Component.java:4716)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.trackMouseEnterExit(Container.java:4620)
at java.awt.LightweightDispatcher.processDropTargetEvent(Container.java:4558)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4417)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:723)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:682)
at java.awt.EventQueue$3.run(EventQueue.java:680)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:696)
at java.awt.EventQueue$4.run(EventQueue.java:694)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:693)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

With assertions off, the exception is the same, minus the first two stack frames.
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