Code review comment for lp:~vanvugt/mir/fix-1194384

One can argue that this whole proposal is unnecessary, except for the docs improvements. Because client authors can use their own locking instead.

The rationale for the proposal then is: Without this, it is impossible to write a reliable, predictable client that handles events without also forcing the client to depend on some third-party locking primitives (pthread, C++11). That's really the issue. With all the features you may choose to include or exclude from an API, determinism is something you need to include.

Also, assuming that all toolkits are thread-aware is a bad assumption. I'm sure there are plenty of toolkits out there that don't have, or care about, threads. And Mir should support them too.