I am running SW2016 and have an issue. Periodically while working in both assemblies and parts, when I switch back to the assembly, the assembly menu won't return. It remains in the part menu. I have to save, exit and return to the assembly to regain the assembly menu? Is this a bug that is being addressed?
I don't remember anyone else with that specific issue, but there have been more people than usual reporting graphics card problems, and that's likely what you're seeing. See #13 at Frequently Asked Forum Questions for more information.