"Fatal error" dialog hangs forever

[mhutch]

The "A fatal error has occurred" dialog reliably hangs for me

Here's a kill -QUIT for the process and an lldb bt for the UI thread: https://gist.github.com/mhutch/fd4ce4c19a7186bb69bffd5e36783d2b

The interesting part is

frame #0: 0x000000010b6deefa libmonosgen-2.0.dylib`mono_lls_find [inlined] mono_memory_barrier at mono-membar.h:67 [opt]
frame #1: 0x000000010b6deefa libmonosgen-2.0.dylib`mono_lls_find [inlined] mono_lls_get_hazardous_pointer_with_mask(pp=0x00007fcd73f19e00, hazard_index=0) at mono-linked-list-set.c:43 [opt]
frame #2: 0x000000010b6deed3 libmonosgen-2.0.dylib`mono_lls_find(list=0x000000010b814498, hp=0x000000010b90e600, key=123145552953344) at mono-linked-list-set.c:97 [opt]
frame #3: 0x000000010b6e0fa7 libmonosgen-2.0.dylib`mono_thread_info_safe_suspend_and_run [inlined] mono_thread_info_lookup(id=<unavailable>) at mono-threads.c:287 [opt]
frame #4: 0x000000010b6e0f8c libmonosgen-2.0.dylib`mono_thread_info_safe_suspend_and_run [inlined] suspend_sync(interrupt_kernel=<unavailable>) at mono-threads.c:950 [opt]
frame #5: 0x000000010b6e0f84 libmonosgen-2.0.dylib`mono_thread_info_safe_suspend_and_run [inlined] suspend_sync_nolock at mono-threads.c:994 [opt]
frame #6: 0x000000010b6e0f84 libmonosgen-2.0.dylib`mono_thread_info_safe_suspend_and_run(id=0x000070000ef08000, interrupt_kernel=1, callback=(libmonosgen-2.0.dylib`async_suspend_critical at threads.c:5024), user_data=0x00007ffee5b00878) at mono-threads.c:1037 [opt]
frame #7: 0x000000010b6228e0 libmonosgen-2.0.dylib`async_suspend_internal(thread=0x0000000125f3f328, interrupt=1) at threads.c:5069 [opt]
frame #8: 0x000000010b62269a libmonosgen-2.0.dylib`mono_thread_suspend_all_other_threads at threads.c:3544 [opt]
frame #9: 0x000000010b5ba580 libmonosgen-2.0.dylib`ves_icall_System_Environment_Exit(result=1) at icall.c:6791 [opt]

VS bug #641294

[mhutch]

I wonder if it's the FSW CFRunLoop