DTL-5956 reports some D&D issues in content panel on Linux.
It's unclear whether those issues are FX bugs or features.
They have been fixed by applying some workaround in content panel.

The same issues seem to applied to Hierarchy panel (see exceptions below).
And similar workaround should also implemented in Hierarchy panel.


Exception #1

Exception in thread "JavaFX Application Thread" java.lang.AssertionError
   at com.oracle.javafx.scenebuilder.kit.editor.drag.DragController.setDropTarget(DragController.java:125)
   at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.HierarchyDNDController.handleOnDragOver(HierarchyDNDController.java:179)
   at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.treeview.HierarchyTreeCell$7.handle(HierarchyTreeCell.java:283)
   at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.treeview.HierarchyTreeCell$7.handle(HierarchyTreeCell.java:273)
   at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
   at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
   at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
   at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
   at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
   at javafx.event.Event.fireEvent(Event.java:203)
   at javafx.scene.Scene$DnDGesture.fireEvent(Scene.java:2867)
   at javafx.scene.Scene$DnDGesture.processTargetEnterOver(Scene.java:3023)
   at javafx.scene.Scene$DnDGesture.access$6100(Scene.java:2843)
   at javafx.scene.Scene$DropTargetListener.dragOver(Scene.java:2774)
   at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:103)
   at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:99)
   at java.security.AccessController.doPrivileged(Native Method)
   at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragOver(GlassSceneDnDEventHandler.java:99)
   at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragOver(GlassViewEventHandler.java:598)
   at com.sun.glass.ui.View.handleDragOver(View.java:668)
   at com.sun.glass.ui.View.notifyDragOver(View.java:986)
   at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystemImpl(Native Method)
   at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystem(GtkDnDClipboard.java:39)
   at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:51)
   at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:59)
   at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:260)
   at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1149)
   at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2887)
   at javafx.scene.Scene$DnDGesture.process(Scene.java:2948)
   at javafx.scene.Scene$DnDGesture.access$8400(Scene.java:2843)
   at javafx.scene.Scene$MouseHandler.process(Scene.java:3675)
   at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3384)
   at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1652)
   at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2443)
   at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:314)
   at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:243)
   at java.security.AccessController.doPrivileged(Native Method)
   at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:345)
   at com.sun.glass.ui.View.handleMouseEvent(View.java:526)
   at com.sun.glass.ui.View.notifyMouse(View.java:898)
   at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
   at com.sun.glass.ui.gtk.GtkApplication.access$200(GtkApplication.java:48)
   at com.sun.glass.ui.gtk.GtkApplication$6$1.run(GtkApplication.java:149)
   at java.lang.Thread.run(Thread.java:744)



Exception #2 (consequence of exception #1 ?)

Exception in thread "JavaFX Application Thread" java.lang.AssertionError
   at com.oracle.javafx.scenebuilder.kit.editor.drag.DragController.setDropTarget(DragController.java:124)
   at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.AbstractHierarchyPanelController.handleOnDragExited(AbstractHierarchyPanelController.java:1053)
   at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.AbstractHierarchyPanelController.access$500(AbstractHierarchyPanelController.java:86)
   at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.AbstractHierarchyPanelController$5.handle(AbstractHierarchyPanelController.java:901)
   at com.oracle.javafx.scenebuilder.kit.editor.panel.hierarchy.AbstractHierarchyPanelController$5.handle(AbstractHierarchyPanelController.java:898)
   at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
   at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
   at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
   at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
   at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
   at javafx.event.Event.fireEvent(Event.java:203)
   at javafx.scene.Scene$DnDGesture.handleExitEnter(Scene.java:3110)
   at javafx.scene.Scene$DnDGesture.processDropEnd(Scene.java:2996)
   at javafx.scene.Scene$DnDGesture.access$6800(Scene.java:2843)
   at javafx.scene.Scene$DragSourceListener.dragDropEnd(Scene.java:3222)
   at com.sun.javafx.tk.quantum.GlassScene$3$1.run(GlassScene.java:249)
   at com.sun.javafx.tk.quantum.GlassScene$3$1.run(GlassScene.java:245)
   at java.security.AccessController.doPrivileged(Native Method)
   at com.sun.javafx.tk.quantum.GlassScene$3.actionPerformed(GlassScene.java:245)
   at com.sun.glass.ui.Clipboard.actionPerformed(Clipboard.java:173)
   at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystem(GtkDnDClipboard.java:41)
   at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:51)
   at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:59)
   at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:260)
   at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1149)
   at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2887)
   at javafx.scene.Scene$DnDGesture.process(Scene.java:2948)
   at javafx.scene.Scene$DnDGesture.access$8400(Scene.java:2843)
   at javafx.scene.Scene$MouseHandler.process(Scene.java:3675)
   at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3384)
   at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1652)
   at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2443)
   at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:314)
   at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:243)
   at java.security.AccessController.doPrivileged(Native Method)
   at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:345)
   at com.sun.glass.ui.View.handleMouseEvent(View.java:526)
   at com.sun.glass.ui.View.notifyMouse(View.java:898)
   at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
   at com.sun.glass.ui.gtk.GtkApplication.access$200(GtkApplication.java:48)
   at com.sun.glass.ui.gtk.GtkApplication$6$1.run(GtkApplication.java:149)
   at java.lang.Thread.run(Thread.java:744)