Make 2D contexts discardable with OffscreenCanvas

[junov]

WebGL has a reasonable model that allows for contexts to be released and rebuilt, which allows, among other things, for GPU resources to be released when a page is in the background. This is implemented via the webglcontextlost and webglcontextrestored events.

The issue has been raised in the past that 2D contexts should have the same feature. However, it is not clear how this could be achieved in a backwards-compatible way. One suggestion was to make 2D contexts discardable only if an event listener is registered on the contextLost event. That idea was discarded because the registration of an event listener is not allowed to have side-effects.

With OffscreenCanvas, we have a clean slate. There is no legacy content to break. Therefore, we could use this opportunity to add the contextLost/restored events to OffscreenCanvases with 2D contexts.

[annevk]

Is @whatwg/canvas still interested in this or can this be closed?

[junov]

Not sure. In Blink, we've explored alternative mitigations, such as using a background task to swap buffers out of GPU memory for browser tabs that are not in the foreground, we're thinking of also png-compressing them. This works kind of okay for preemptively freeing resources. That said, there is still no good solution for detecting and recovering from an unforeseen GPU reset. These GPU resets are not as frequent as they used to be thanks to huge improvements to display drivers and OSes.

[annevk]

I guess we can still consider this a while longer given that only Safari is shipping OffscreenCanvas and only for "webgl".