JNI DetachCurrentThread decrements the UserThreadCount while
JNI AttachCurrentThread doesn't - under green threads. On
native & win32 AttachCurrentThread increments UserThreadCount.

One effect of this is that java_main.c which does the following JNI
calls in sequence: DetachCurrentThread, AttachCurrentThread, DestroyJavaVM -
exits prematurely because the UserThreadCount is incorrectly one. This, at
a minimum, causes the debugger to exit prematurely.