SDL2 application will break when using pthreads

I have made a SDL2 application using the linker flag "-s USE_SDL=2" which works as expected.
I have made a ASIO application using https://github.com/emscripten-ports/asio which works as expected.
I have made a ASIO/pthread application with works as expected using the linker flags "-s USE_PTHREADS=1 -s PTHREAD_POOL_SIZE=4 -s PROXY_TO_PTHREAD=1"

But I can not merge these applications together. It seems as if USE_SDL and USE_PTHREADS/PROXY_TO_PTHREAD works against each other. I known that many older tutorials states that multithreading is not possible, but also knows that a lot of activities has been done to change this.

I get the following error when combining SDL and pthreads:
pthread-main.js onmessage() captured an uncaught exception: ReferenceError: screen is not defined

I can understand that I would make sense that the resources for rendering would only be available for the main thread, but pthread is missing some features/resources for SDL to possible.

Is it by design that USE_SDL and USE_PTHREADS/PROXY_TO_PTHREAD work against each other?
Would it be possible to use SDL from the main thread and still use pthreads for additional work?