Lighthouse: "Uses passive event listeners to improve scrolling performance" documentation needs updating

[kleinfreund]

On the Uses Passive Event Listeners to Improve Scrolling Performance page it states that the implementation "Filter[s] out listeners that call preventDefault()".

This doesn’t seem to work properly as the following listener makes it into the report when Lighthouse reports a failed Uses passive listeners to improve scrolling performance.

```
document.addEventListener('wheel', function(event) {
event.preventDefault();
})
````

I expected the case to pass since all wheel event listeners in my code call preventDefault. Maybe I missed something here.

• Google Chrome version 60.0.3112.101
• Lighthouse version 2.3.0

[patrickhulce]

Ah thanks for the report @kleinfreund the description is out of date (cc: @kaycebasques). This audit is now backed by Chrome's built-in performance violations which flags the use of these listeners in any case, not just ones that could be marked as passive without any code change.

[paulirish]

This was also just raised here: https://bugs.chromium.org/p/chromium/issues/detail?id=489802#c42

I think its fairly reasonable for us to not warn you if you set {passive:false}. We will still need a documentation change in this case.

But we'll also need a chrome-side change.

[tdresser]

@dtapuska - do you think we should warn on use of passive:false?

[dtapuska]

Ya I agree we shouldn't warn; 770208 opened

[kul3r4]

I can also see this error when using Polymer components, like in this demo.

[dtapuska]

Yes that is because polymer by default doesn't use passive touch events. You need to enable passiveTouchGestures setting in polymer.

[kul3r4]

Thanks!
Is this available in Polymer 2? I tried to add it in this way in the index.html:

    <link rel="import" href="/polymer/node_modules/@npm-polymer/polymer/polymer.html">
    <script>
        Polymer.setPassiveTouchGestures(true);
    </script>

but I get:

Uncaught TypeError: Polymer.setPassiveTouchGestures is not a function

[dtapuska]

Look at https://github.com/Polymer/polymer/issues/4847

[kul3r4]

Thanks, it says to

move the script down below the Polymer import.

which is what I am doing, but I still see the error, do you know of a complete working sample using it?

[dtapuska]

Are you using version 2.1.0 or later?

[kul3r4]

I am using 2.0.1

[kleinfreund]

I think this can be closed, right?

[paulirish]

Yes. If you set {passive:false} you will not get a console violation nor will Lighthouse flag it.