After updating to SDL 2.0.9, I got a user report that my app was crashing when closing a SDL_WINDOW_FULLSCREEN window to return to my Qt-based UI. It looks like the dead SDL window is getting a spurious updateLayer call which is causing SDL to dereference a null SDL_WindowData pointer.

For some reason, this only happens when using SDL_WINDOW_FULLSCREEN and not windowed or SDL_WINDOW_FULLSCREEN_DESKTOP. I was also unsuccessful in my attempt to get a simple reproducer for this crash. The Session.cpp code is available https://github.com/moonlight-stream/moonlight-qt/blob/688c4a90d994aa23e7b0af3ffcbb8707886db780/app/streaming/session.cpp but I slightly modified it (adding a SDL_PumpEvents() call at 1179 to immediately trigger the issue, otherwise it happened when Qt next pumped the event loop).

The crashing line is:

    NSMutableArray *contexts = data->nscontexts;

The following is a symbolicated crash using the latest HG code available (144400e4630d885d2eb0761b7174433b4c0d90bb).

Process:               Moonlight [47354]
Path:                  /Users/USER/*/Moonlight.app/Contents/MacOS/Moonlight
Identifier:            Moonlight
Version:               0
Code Type:             X86-64 (Native)
Parent Process:        Qt Creator [46471]
Responsible:           Moonlight [47354]
User ID:               501

Date/Time:             2018-11-19 20:34:50.175 -0800
OS Version:            Mac OS X 10.14.1 (18B75)
Report Version:        12
Anonymous UUID:        9CDFA65F-02F8-9ED1-4583-E823280598DA


Time Awake Since Boot: 80000 seconds

System Integrity Protection: disabled

Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Exception Codes:       KERN_INVALID_ADDRESS at 0x0000000000000010
Exception Note:        EXC_CORPSE_NOTIFY

Termination Signal:    Segmentation fault: 11
Termination Reason:    Namespace SIGNAL, Code 0xb
Terminating Process:   exc handler [47354]

VM Regions Near 0x10:
--> 
    __TEXT                 0000000103d62000-0000000104011000 [ 2748K] r-x/rwx SM=COW  /Users/USER/*/Moonlight.app/Contents/MacOS/Moonlight

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   libSDL2-2.0.0.dylib           	0x00000001046a7c30 ScheduleContextUpdates + 64 (SDL_cocoawindow.m:228)
1   libSDL2-2.0.0.dylib           	0x00000001046a98ea -[SDLView updateLayer] + 90 (SDL_cocoawindow.m:1190)
2   com.apple.AppKit              	0x00007fff4a5056c7 _NSViewUpdateLayer + 45
3   com.apple.AppKit              	0x00007fff4a5050e7 -[_NSViewBackingLayer display] + 505
4   com.apple.QuartzCore          	0x00007fff57e5d1a4 CA::Layer::display_if_needed(CA::Transaction*) + 634
5   com.apple.QuartzCore          	0x00007fff57e4b42f CA::Context::commit_transaction(CA::Transaction*) + 319
6   com.apple.QuartzCore          	0x00007fff57e4ad00 CA::Transaction::commit() + 576
7   com.apple.AppKit              	0x00007fff4a4fbe41 __65+[CATransaction(NSCATransaction) NS_setFlushesWithDisplayRefresh]_block_invoke + 274
8   com.apple.CoreFoundation      	0x00007fff4cf016ad __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
9   com.apple.CoreFoundation      	0x00007fff4cf015e2 __CFRunLoopDoObservers + 452
10  com.apple.CoreFoundation      	0x00007fff4cea3109 __CFRunLoopRun + 1166
11  com.apple.CoreFoundation      	0x00007fff4cea2a28 CFRunLoopRunSpecific + 463
12  com.apple.HIToolbox           	0x00007fff4c13bb35 RunCurrentEventLoopInMode + 293
13  com.apple.HIToolbox           	0x00007fff4c13b774 ReceiveNextEventCommon + 371
14  com.apple.HIToolbox           	0x00007fff4c13b5e8 _BlockUntilNextEventMatchingListInModeWithFilter + 64
15  com.apple.AppKit              	0x00007fff4a3f7eb7 _DPSNextEvent + 997
16  com.apple.AppKit              	0x00007fff4a3f6c56 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 1362
17  libSDL2-2.0.0.dylib           	0x00000001046a181a Cocoa_PumpEvents + 138 (SDL_cocoaevents.m:438)
18  libSDL2-2.0.0.dylib           	0x00000001045f33a7 SDL_PumpEvents_REAL + 23 (SDL_events.c:657)
19  Moonlight                     	0x0000000103dbb847 Session::exec(int, int) + 6087 (session.cpp:1179)