I think I have found the problem: SDL2 for window creation.Now I implemented my own window manager with contextual graphics bindings for Vulkan, OpenGL and DirectX (9/10/11/12)...
It seems like it can complicate things when you want to use multiple threads for rendering and window management. I hope that your own solution will overcome the limitations of SDL2 with messaging and stateful control. Good luck...
SDB@79
