testmessage failure on macOS

[slouken]

Running testmessage fails on the threaded messagebox test on macOS:

2021-06-04 13:45:44.199 testmessage[32192:1275778] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'NSWindow drag regions should only be invalidated on the Main Thread!'
*** First throw call stack:
(
	0   CoreFoundation                      0x000000018f7f6320 __exceptionPreprocess + 240
	1   libobjc.A.dylib                     0x000000018f524c04 objc_exception_throw + 60
	2   CoreFoundation                      0x000000018f8206d0 _CFBundleGetValueForInfoKey + 0
	3   AppKit                              0x0000000191f7be4c -[NSWindow(NSWindow_Theme) _postWindowNeedsToResetDragMarginsUnlessPostingDisabled] + 380
	4   AppKit                              0x0000000191f65ebc -[NSWindow _initContent:styleMask:backing:defer:contentView:] + 916
	5   AppKit                              0x00000001921210d8 -[NSPanel _initContent:styleMask:backing:defer:contentView:] + 48
	6   AppKit                              0x0000000191f65b1c -[NSWindow initWithContentRect:styleMask:backing:defer:] + 56
	7   AppKit                              0x000000019212108c -[NSPanel initWithContentRect:styleMask:backing:defer:] + 48
	8   AppKit                              0x0000000191f63ef8 -[NSWindowTemplate nibInstantiate] + 292
	9   AppKit                              0x0000000191f2d954 -[NSIBObjectData instantiateObject:] + 236
	10  AppKit                              0x0000000191f2d1e0 -[NSIBObjectData nibInstantiateWithOwner:options:topLevelObjects:] + 400
	11  AppKit                              0x0000000191f21744 loadNib + 376
	12  AppKit                              0x0000000191f20ccc +[NSBundle(NSNibLoading) _loadNibFile:nameTable:options:withZone:ownerBundle:] + 756
	13  AppKit                              0x0000000191f208fc -[NSBundle(NSNibLoading) loadNibNamed:owner:topLevelObjects:] + 220
	14  AppKit                              0x00000001922ab284 -[NSAlert init] + 156
	15  libSDL2-2.0.0.dylib                 0x0000000100fac298 Cocoa_ShowMessageBox + 64
	16  libSDL2-2.0.0.dylib                 0x0000000100f9d22c SDL_ShowMessageBox_REAL + 448
	17  testmessage                         0x0000000100ed7ac4 button_messagebox + 140
	18  libSDL2-2.0.0.dylib                 0x0000000100f5b4d8 SDL_RunThread + 48
	19  libSDL2-2.0.0.dylib                 0x0000000100fb99dc RunThread + 12
	20  libsystem_pthread.dylib             0x000000018f67e06c _pthread_start + 320
	21  libsystem_pthread.dylib             0x000000018f678da0 thread_start + 8
)
libc++abi.dylib: terminating with uncaught exception of type NSException

[icculus]

Yikes, I think we could try dispatching it to the main thread, but I assume this won't work from the background thread ever. I'll research it.

[slouken]

Fixed by 51c61d7