[jbroman]

> Maybe browsers could do some static analysis for the simple cases?

The risk here is that you end up where cases where non-obvious changes cause the static analysis to no longer work; at least in the current world there's a clear line between having a non-passive event listener and not having one. Attempting a heuristic could cause both unpredictability and interoperability issues (since the heuristic will surely vary by browser vendor).