Incompatibility of Editor under OS X 10.10
I just realized while building 4.7 Preview myself on OS X 10.10, there seems to be an with open gl window being added as sub view to super view of content window. following exception arises:
2015-01-20 23:20:54.643 UE4Editor-Mac-Debug[30748:8226068] NSWindow warning: adding an unknown subview: <FCocoaOpenGLView: 0x6180001768c0>
2015-01-20 23:20:54.663 UE4Editor-Mac-Debug[30748:8226068] Call stack:
(
0 AppKit 0x00007fff9674c1ec -[NSThemeFrame addSubview:] + 107
1 UE4Editor-OpenGLDrv-Mac-Debug.dylib 0x000000012cf4e153 _Z27PlatformCreateOpenGLContextP21FPlatformOpenGLDevicePv + 1363
2 UE4Editor-OpenGLDrv-Mac-Debug.dylib 0x000000012cf4dae7 _ZN15FOpenGLViewportC2EP17FOpenGLDynamicRHIPvjjb + 967
3 UE4Editor-OpenGLDrv-Mac-Debug.dylib 0x000000012cf45f44 _ZN15FOpenGLViewportC1EP17FOpenGLDynamicRHIPvjjb + 68
4 UE4Editor-OpenGLDrv-Mac-Debug.dylib 0x000000012cf45eb7 _ZN17FOpenGLDynamicRHI17RHICreateViewportEPvjjb + 167
5 UE4Editor-RHI-Mac-Debug.dylib 0x0000000108f0b6a5 _Z17RHICreateViewportPvjjb + 165
6 UE4Editor-SlateRHIRenderer-Mac-Debug.dylib 0x00000001217052b5 _ZN17FSlateRHIRenderer14CreateViewportE10TSharedRefI7SWindowL7ESPMode0EE + 2293
7 UE4Editor-SlateCore-Mac-Debug.dylib 0x0000000109db8192 _ZN7SWindow10ShowWindowEv + 722
8 UE4Editor-MainFrame-Mac-Debug.dylib 0x000000010dfa33c6 _ZNK17FMainFrameHandler19ShowMainFrameWindowE10TSharedRefI7SWindowL7ESPMode0EEb + 646
9 UE4Editor-MainFrame-Mac-Debug.dylib 0x000000010df586c1 _ZN16FMainFrameModule22CreateDefaultMainFrameEb + 13137
10 UE4Editor-UnrealEd-Mac-Debug.dylib 0x000000010b326e2a _Z10EditorInitR11IEngineLoop + 13002
11 UE4Editor-Mac-Debug 0x0000000100044c2a _Z11GuardedMainPKw + 138
12 UE4Editor-Mac-Debug 0x0000000100059db9 -[UE4AppDelegate runGameThread:] + 297
13 UE4Editor-Core-Mac-Debug.dylib 0x00000001003065fb -[FCocoaGameThread ] + 123
14 Foundation 0x00007fff98b7590a __NSThread__main__ + 1345
15 libsystem_pthread.dylib 0x00007fff94c3a268 _pthread_body + 131
16 libsystem_pthread.dylib 0x00007fff94c3a1e5 _pthread_body + 0
17 libsystem_pthread.dylib 0x00007fff94c3841d thread_start + 13
)
For now, seems to be only a warning, cause I do not see anything that does not work. Apples documentation says, that starting with Yosemite, it shall no longer be possible to use NSWindow’s addSubView method for purpose (see OpenGLMac.cpp). Maybe someone of your Mac gurus can figure something out for 
Regards,
Heiko