Code review comment for lp:~albaguirre/mir/fix-1499039

Nits:

Seems to upset vivid builds. Because I don't think you mean this:

+ auto comp1{reinterpret_cast<mc::CompositorID>(0)};
+ auto comp2{reinterpret_cast<mc::CompositorID>(1)};

But

    auto const comp1 = reinterpret_cast<mc::CompositorID>(0);
    auto const comp2 = reinterpret_cast<mc::CompositorID>(1);

Using "auto" with initializer lists is a Bad Idea as it rarely does what you intend.

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+ mutable std::mutex guard;

cv-qualifiers should follow the type