OffscreenCanvasRenderingContext2D does not have getContextAttributes

[ccameron-chromium]

What is the issue with the HTML Standard?

The getContextAttributes method allows for querying properties of a CanvasRenderingContext2D, such as its color space, alpha state, etc.

This method is not present on OffscreenCanvasRenderingContext.

This doesn't seem right, especially considering that the offscreen canvas 2D context is created using CanvasRenderingContext2DSettings`. Is that just an oversight?

[annevk]

Seems worth looking at git blame (see README) to see if there was a reason. Might just be MVP.

cc @whatwg/canvas

[schenney-chromium]

Interestingly there is no discussion of what the options are for OffscreenCanvas.getContext.

[ccameron-chromium]

It dates back to this issue (sorry, forgot to mention it). It has a funny history where the API was accidentally shipped, then a Blink "Intent to Remove" caused people to realized that maybe we need it. It was committed here.

Do you think adding this would count as a clarification, or should it have a full review? Would it be best to fix this by:

• Option A: Creating a new mixin common to CanvasRenderingContext2D and OffscreenCanvasRenderingContext2D
• Option B: Adding a new method on OffscreenCanvasRenderingContext2D

I'm leaning towards Option B (since most related functionality, like 2d context creation, is separate).

[ccameron-chromium]

To fully link things:

• The 2D context creation algorithm for HTMLCanvasElement is here
• The offscreen 2D context creation algorithm is here

They share alpha and colorSpace. They do not share desynchronized or willReadFrequently.

[annevk]

All normative changes go through this process: https://whatwg.org/working-mode#changes

I.e., PR + tests + implementer interest.

B seems fine. We should also split the dictionary if we don't actually support the same options, to enable feature testing going forward.