Adding passive event listeners

[a1626]

Description

DevTool of chrome canary in verbose mode gives a violation

[Violation] Added non-passive event listener to a scroll-blocking 'touchstart' event. Consider marking event handler as 'passive' to make the page more responsive.

This violation comes for touchstart and mousewheel events

Steps to Reproduce

1. Create a small demo application using Polymer
2. Open the application in chrome canary
3. In console of devTool switch level to verbose.

Versions

• Polymer: v1.X.X
• webcomponents: vX.X.X

[khammami]

I analyzed my app with Lighthouse

it suggest to uses passive listeners to improve scrolling performance.

[wingliu0]

@khammami I got the same warning too. But the one adding non-passive listeners is polymer itself.. and some iron component. How should we replace that?

[khammami]

Hi @wingliu0, here what I found about passive events. touch listeners should be annotated with {passive: true}. I believe the best way to replace them is by reporting each non passive listener you find to the team.

[wingliu0]

@khammami true, but will they still maintain the polymer 1.x?

[khammami]

@wingliu0 I think so, at least till the final release of v2

[ebidel]

Dupe of #3604