Worked: original patch from AMD for split kernel if I remember correctly.
Short description of error
When adding hairs for example while in viewport render, the kernel will not recompile, making the hairs invisible. User has to toggle twice the render mode for the viewport to get it display the scene correctly. No hints is given about what is wrong.
Exact steps for others to reproduce the error
Start viewport render on the default cube scene and add a hair system. It doesn't render. Toggling to solid mode and back to rendered mode solves the problem.