Polymer thoughts on passive event listeners?

[Rick Byers]

Hey guys,

I realized I've been missing a great source of valuable information by not always soliciting feedback from the polymer team on web platform changes we're working on.

So here's one I'd love your input on: passive event listeners. Daniel, I'm sure this will raise all sorts of emotions for you wrt. pointer events (basically it's designed to bring the good perf properties of pointer events to touch and wheel events).  Are there scenarios today where Polymer is blocking scroll unnecessarily via it's event listeners?  Could you, in theory, make ALL your touch and wheel listeners 'passive'?  Any feedback on the API or solutions you think would be better?

Feel free to file issues on GitHub if you like.

Thanks,

   Rick

[Daniel Freedman]

+ egarciad, cdata

I think most of the touch listeners for polymer could be made passive, except for the one handler that emulates touch-action: none|pan-x|pan-y.

Chris, Emmanuel, are there any other listeners (mousewheel, scroll, etc) you think might be made passive?

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[Emmanuel Garcia]

1. paper-slider uses preventDefault to prevent selection. https://github.com/Polymer/paper-slider/blob/master/paper-slider.html#L305-L311  Technically speaking the workaround is to set -webkit-user-select:none to the document element, but that may require coordination.

2. paper-drawer-panel for the same reason as paper-slider: https://github.com/PolymerElements/paper-drawer-panel/blob/master/paper-drawer-panel.html#L658

3.  Polymer.IronDropdownScrollManager owned by Chris.

~ Emmanuel

[Rick Byers]

On Thu, Jan 7, 2016 at 7:57 PM, Emmanuel Garcia <egar...@google.com> wrote:

1. paper-slider uses preventDefault to prevent selection. https://github.com/Polymer/paper-slider/blob/master/paper-slider.html#L305-L311  Technically speaking the workaround is to set -webkit-user-select:none to the document element, but that may require coordination.

Is this for mouse or touch or both?  passive listeners don't really apply to mousedown/mousemove etc since there's no conflict there between an app action and scrolling (only 'wheel' and touch events have this problem).

2. paper-drawer-panel for the same reason as paper-slider: https://github.com/PolymerElements/paper-drawer-panel/blob/master/paper-drawer-panel.html#L658

3.  Polymer.IronDropdownScrollManager owned by Chris.

~ Emmanuel

On Thu, Jan 7, 2016 at 2:14 PM, Daniel Freedman <dfr...@chromium.org> wrote:

+ egarciad, cdata

I think most of the touch listeners for polymer could be made passive, except for the one handler that emulates touch-action: none|pan-x|pan-y.

Yep, that makes sense.  Do you still use that handler on browsers with native touch-action support (Chrome, IE, Edge)?

Chris, Emmanuel, are there any other listeners (mousewheel, scroll, etc) you think might be made passive?

Note that scroll events are always uncancelable (async), so not an issue here. 

[Emmanuel Garcia]

Is this for mouse or touch or both?  passive listeners don't really apply to mousedown/mousemove etc since there's no conflict there between an app action and scrolling (only 'wheel' and touch events have this problem).

I believe the `down` event is fired from `toushstart` on mobile. +dfreedm for confirmation.

https://github.com/Polymer/polymer/blob/273ab0fbe641209fa3fc3ce8368bb78f83db9b10/src/standard/gestures.html#L451

Other than that, the only one I can see that may want to block scrolling is IronDropdownScrollManager  https://github.com/PolymerElements/iron-dropdown/blob/master/iron-dropdown-scroll-manager.html#L181  +cdata

Thanks

~ Emmanuel