Xterm.js: Violation warning about touch and wheel event listeners not being passive

[parisk]

It seems that Chrome does not like that we do not use passive event listeners for touch and wheel events.

More information on passive event listeners:

• https://developers.google.com/web/updates/2016/06/passive-event-listeners
• https://github.com/WICG/EventListenerOptions/blob/gh-pages/explainer.md
• https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#Improving_scrolling_performance_with_passive_listeners

Details

• Browser and browser version: Chrome 59
• OS version: macOS Sierra
• xterm.js version: 2.8.1

Steps to reproduce

1. Open demo
2. Open console to view logs

Screenshot

[Tyriar]

Do we just need to mark it as { passive: false } as we need to cancel the scroll currently?

[mofux]

@Tyriar I think that is what needs to be done, yes. I believe it is only warning because we have not explicitly told the handler how to behave.

[Tyriar]

I can't seem to get the warning in my console so this is tough for me to repro, I have definitely seen it before though.

[mofux]

As far as I can see we never prevent the scroll event, which means we should be able to safely register it as a passive handler via { passive: true }.

[Tyriar]

This one's driving me crazy, I don't want to fix it unless I can actually reproduce the warning but can't seem to on any of my machines 😕

[nikonso]

@Tyriar, @mofux

Hey, I manged to repro the warning on my machine, but while trying to fix it, I came to the conclusion that we can't mark them all as passive because of this block of code, where we're preventing page scrolling.

https://github.com/xtermjs/xterm.js/blob/a8964a15ddf8d2e09aca51cf29b0f83440ef5ea2/src/Viewport.ts#L125-L127

What should I do, ignore this one and mark the others as passive, do we ignore this issue, or a third option maybe?

PS: If we explicitly set passive to false the warning also seems to disappear...

[Tyriar]

@nikonso yes we want to explicitly say that they are not passive, that will go away with the warning as we're telling the browser that we know about the risks.