On connecting to a second screen, Unity Editor will generate several errors, exceptions and asserts in the console.

1. Have the editor running.
2. Connect to a second screen.

Notice that the following appear:

GUI Window tried to begin rendering while something else had not finished rendering! Either you have a recursive OnGUI rendering, or the previous OnGUI did not clean up properly.

InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Pop () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections/Stack.cs:329)
UnityEngine.GUILayoutUtility.EndLayoutGroup () (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:323)
UnityEngine.Experimental.UIElements.UIElementsUtility.EndContainerGUI () (at C:/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:174)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:109)
UnityEngine.Experimental.UIElements.IMGUIContainer.HandleEvent (UnityEngine.Event evt, UnityEngine.Experimental.UIElements.VisualElement finalTarget) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:137)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoRepaint (IStylePainter args) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:35)
UnityEngine.Experimental.UIElements.Panel.PaintSubTree (UnityEngine.Event e, UnityEngine.Experimental.UIElements.VisualElement root, Matrix4x4 offset, Rect currentClip, Matrix4x4 clipTransform, Rect currentGlobalClip) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/Panel.cs:374)
UnityEngine.Experimental.UIElements.Panel.PaintSubTree (UnityEngine.Event e, UnityEngine.Experimental.UIElements.VisualElement root, Matrix4x4 offset, Rect currentClip, Matrix4x4 clipTransform, Rect currentGlobalClip) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/Panel.cs:382)
UnityEngine.Experimental.UIElements.Panel.Repaint (UnityEngine.Event e) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/Panel.cs:393)
UnityEngine.Experimental.UIElements.UIElementsUtility.DoDispatch (IVisualElementPanel panel) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:214)
UnityEngine.Experimental.UIElements.UIElementsUtility.ProcessEvent (Int32 instanceID, IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:67)
UnityEngine.GUIUtility.ProcessEvent (Int32 instanceID, IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUIUtility.cs:186)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Assertion failed: Assertion failed on expression: 'device.IsInsideFrame()'

Invalid GUIClip stack popping
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)