How to reproduce:

const app = new App({
    skipAuth: true,  // not needed, just for easier local testing
});

app.on("message", async ({ send }) => {
    await send("Hello there!");
});
app.start();

With the above setup, activities of type message will be handled as expected by the handler we defined via app.on. However, for any other kind of activity (like, for example, installationUpdate) the request will hang indefinitely.
I believe the cause is here: