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+Ensure a `VectorDiff` is not emitted to subscribers when an event cache is
+shrunk because no subscribers is listening to it. If there is no subscribers,
+it's useless to send it anyway, and if in the future, a race happens and a
+subscriber exists, it must not receive this diff.
+
+At the same time, the way the subscribers are tracked and counted have been
+revisited to use less space in memory and use less atomic operations.