To Reproduce:

1. Open any Unity project on macOS project.
2. Observe that in Editor.log:

2017-03-03 13:26:57.371 Unity[90009:12994172] NSWindow warning: adding an unknown subview: <NSView: 0x600000127940>. Break on NSLog to debug.
2017-03-03 13:26:57.397 Unity[90009:12994172] Call stack:
(
0 AppKit 0x00007fff82aab67d -[NSThemeFrame addSubview:] + 109
1 AppKit 0x00007fff82aab3c8 -[NSView addSubview:positioned:relativeTo:] + 217
2 AppKit 0x00007fff832c1011 -[NSThemeFrame addSubview:positioned:relativeTo:] + 43
3 Unity 0x0000000102fe711f _ZN15ContainerWindow4InitEP13MonoBehaviour5RectTIfEiRK8Vector2fS6_ + 1647
4 Unity 0x000000010290670a _Z50ContainerWindow_CUSTOM_INTERNAL_CALL_Internal_ShowP10MonoObjectRK5RectTIfEiRK13Vector2fIcallS7_ + 298
5 ??? 0x00000001482c2add 0x0 + 5505821405
6 ??? 0x0000000147a23926 0x0 + 5496781094
7 libmono.0.dylib 0x000000010e662b52 mono_jit_runtime_invoke + 1766
8 libmono.0.dylib 0x000000010e78d81e mono_runtime_invoke + 117
9 Unity 0x0000000102083e4b _ZN19ScriptingInvocation6InvokeEP21ScriptingExceptionPtrb + 667
10 Unity 0x000000010038b1dd _Z17LoadCurrentLayoutb + 269
11 Unity 0x000000010038a042 _Z28LoadDefaultWindowPreferencesv + 754
12 Unity 0x0000000102ea30c5 _ZN11Application20FinishLoadingProjectEv + 885
13 Unity 0x0000000102fd3a3d -[EditorApplication applicationDidFinishLaunching:] + 909
14 CoreFoundation 0x00007fff84f75a6c __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 12
15 CoreFoundation 0x00007fff84f7596b _CFXRegistrationPost + 427
16 CoreFoundation 0x00007fff84f756d2 ___CFXNotificationPost_block_invoke + 50
17 CoreFoundation 0x00007fff84f32d63 -[_CFXNotificationRegistrar find:object:observer:enumerator:] + 1827
18 CoreFoundation 0x00007fff84f31d9c _CFXNotificationPost + 604
19 Foundation 0x00007fff86958a37 -[NSNotificationCenter postNotificationName:object:userInfo:] + 66
20 AppKit 0x00007fff82bd32b5 -[NSApplication _postDidFinishNotification] + 297
21 AppKit 0x00007fff82bd301a -[NSApplication _sendFinishLaunchingNotification] + 208
22 AppKit 0x00007fff82a96383 -[NSApplication(NSAppleEventHandling) _handleAEOpenEvent:] + 552
23 AppKit 0x00007fff82a95fd5 -[NSApplication(NSAppleEventHandling) _handleCoreEvent:withReplyEvent:] + 661
24 Foundation 0x00007fff869a3ebd -[NSAppleEventManager dispatchRawAppleEvent:withRawReply:handlerRefCon:] + 290
25 Foundation 0x00007fff869a3d37 _NSAppleEventManagerGenericHandler + 102
26 AE 0x00007fff85da60da _Z20aeDispatchAppleEventPK6AEDescPS_jPh + 544
27 AE 0x00007fff85da5e51 _ZL25dispatchEventAndSendReplyPK6AEDescPS_ + 39
28 AE 0x00007fff85da5d5d aeProcessAppleEvent + 312
29 HIToolbox 0x00007fff844fc74f AEProcessAppleEvent + 55
30 AppKit 0x00007fff82a9187d _DPSNextEvent + 1833
31 AppKit 0x00007fff8320bd6b -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 2789
32 AppKit 0x00007fff82a85f35 -[NSApplication run] + 926
33 AppKit 0x00007fff82a50850 NSApplicationMain + 1237
34 Unity 0x0000000102ff763a _Z10EditorMainiPPKc + 2010
35 Unity 0x0000000102ff7dc9 main + 9
36 Unity 0x00000001000025a4 start + 52
37 ??? 0x0000000000000001 0x0 + 1)

It doesn't seem to have any actual effect.