Html: Add a low latency mode for 2D/3D Canvas contexts

[yellowdoge]

@junov filed #2659 for OffscreenCanvas. This is a sister issue for on-screen canvas (2d/3d), and is needed as a companion for Chromium development happening in http://crbug.com/788439 (mostly for ChromeOS since there we control the OS Compositor).

The original proposal from @junov stays the same with s/singleBuffered/lowLatency/, i.e. add a

partial dictionary CanvasContextCreationAttributes {
  boolean lowLatency = false;
}

+cc @fserb, @wffurr, @beaufortfrancois

[annevk]

@whatwg/canvas

[yellowdoge]

For future reference, one of my concerns was how to communicate back to the app if/when lowLatency is supported but @kenrussell brought to my attention that getContextAttributes() returns the _actual_ context parameters, and this can be used. I haven't found a similar readback mechanism on 2D canvas contexts though.

[annevk]

That's #2563, but hasn't been touched in a while.

[domenic]

Is anyone considering working on a pull request for this? And/or for #2563?

[wffurr]

There's an origin trial for this in Chrome 71: https://www.chromestatus.com/feature/6360971442388992

[domenic]

I sure hope nobody is considering shipping it without a spec.

[domenic]

Are other implementers interested in this? There's a spec up at https://github.com/whatwg/html/pull/4234.

[yellowdoge]

@jan-ivar from FF to say how they like this Spec proposal/ forward to some other interested FF devs

[jan-ivar]

I'll defer to @annevk here.

[kenrussell]

@jdashg from Mozilla has given good feedback on this proposal in the WebGL working group.

[domenic]

Thanks, is there any public (e.g. minuted) sign of implementation interest we can link to?

[annevk]

@jrmuizel might also be able to comment on behalf of Mozilla.

[smfr]

I'd like the spec to say a bit more about how "desynchronized" affects event handling, if at all. Presumably the page still has to run JS to handle pointer events and call canvas API; does that part follow the normal HTML event loop rules? If so, isn't canvas update rate still limited by how responsive the event loop is?

[annevk]

This should have been closed by #4360. Filed #4865 for @smfr's comment.

[aminya]

How can I know that the lowLatency or unsynchronized features are supported?

On Ubuntu, setting these values to true causes problems.
https://github.com/atom-minimap/minimap/issues/724

[jdashg]

getContextAttributes() will tell you if context creation attributes were actually applied.
The issues there look like a browser bug though, and should be filed into the affected browser's bug reporting system.