gl_only_in_pthread.cpp error

https://github.com/emscripten-core/emscripten/blob/main/test/gl_only_in_pthread.cpp

the compile script:
`emcc gl_only_in_pthread.cpp -o gl_only_in_pthread.html -s USE_PTHREADS=1 -s PTHREAD_POOL_SIZE=2 -s OFFSCREENCANVAS_SUPPORT=1 -lGL -s OFFSCREEN_FRAMEBUFFER=1`

we run the gl_only_in_pthread.html , we should see the WebGL canvas fade from black to red. but In fact, After the thread exits, the canvas changes from black to red, and there is no gradient process. so why i can't see fade from black to red. is the compile script error ?