Many test cases and applications seem to assume both. This is worth providing even a partial implementation just to provide test coverage.

This bug tracks putting together an initial implementation with the caveat that text rendering may be missing or suboptimal given its complexity.

It appears for fonts, we need more than just OffscreenCanvasRenderingContext2D. FontFace, etc are expected to be usable on a worker thread.

This patch is a non-functional change which allows the shutdown observer
to be overridden (since workers have a different mechanism). It also
fixes a static counter to be atomic, and adds mOffscreenCanvas to the
cycle collection tracking.

This patch is a non-functional change. It overloads some WebIDL methods
of CanvasRenderingContext2D to account for the difference between the
main thread and worker prototypes.

Depends on D135352

This patch adds a partial OffscreenCanvasRenderingContext2D
implementation. It is missing anything text and UI related, including
CanvasFilters, CanvasUserInterface, CanvasText, CanvasTextDrawingStyles,
and CanvasHitRegions.

Depends on D135353

Backed out for causing mochitest webgl failures on test_2_conformance2__offscreencanvas__offscreencanvas-sync.html.
Later, more failures appeared:
This linux build bustage
This wpt failures
and This Windows MinGW all build bustages

Push with failures

Failure log

Backout link

[task 2022-02-02T21:17:25.382Z] 21:17:25     INFO - TEST-START | dom/canvas/test/webgl-conf/generated/test_2_conformance2__offscreencanvas__offscreencanvas-sync.html
[task 2022-02-02T21:17:29.020Z] 21:17:29     INFO - GECKO(1326) | 2022-02-02 21:17:29.019 firefox[1326:15595] Persistent UI failed to open file file:///Users/cltbld/Library/Saved%20Application%20State/org.mozilla.nightly.savedState/window_1.data: No such file or directory (2)
[task 2022-02-02T21:22:45.630Z] 21:22:45     INFO - TEST-INFO | started process screencapture
[task 2022-02-02T21:22:45.749Z] 21:22:45     INFO - TEST-INFO | screencapture: exit 0
[task 2022-02-02T21:22:45.749Z] 21:22:45     INFO - Buffered messages logged at 21:17:25
[task 2022-02-02T21:22:45.750Z] 21:22:45     INFO - TEST-PASS | dom/canvas/test/webgl-conf/generated/test_2_conformance2__offscreencanvas__offscreencanvas-sync.html | A valid string reason is expected 
[task 2022-02-02T21:22:45.750Z] 21:22:45     INFO - TEST-PASS | dom/canvas/test/webgl-conf/generated/test_2_conformance2__offscreencanvas__offscreencanvas-sync.html | Reason cannot be empty 
[task 2022-02-02T21:22:45.750Z] 21:22:45     INFO - Buffered messages finished
[task 2022-02-02T21:22:45.751Z] 21:22:45     INFO - TEST-UNEXPECTED-FAIL | dom/canvas/test/webgl-conf/generated/test_2_conformance2__offscreencanvas__offscreencanvas-sync.html | Test timed out. - 
[task 2022-02-02T21:22:46.633Z] 21:22:46     INFO - GECKO(1326) | MEMORY STAT | vsize 6608MB | residentFast 98MB | heapAllocated 11MB
[task 2022-02-02T21:22:46.638Z] 21:22:46     INFO - TEST-OK | dom/canvas/test/webgl-conf/generated/test_2_conformance2__offscreencanvas__offscreencanvas-sync.html | took 321256ms
[task 2022-02-02T21:22:46.647Z] 21:22:46     INFO - TEST-START | dom/canvas/test/webgl-conf/generated/test_2_conformance2__offscreencanvas__offscreencanvas-timer-query.html

Single failing test on OSX opt (not debug interestingly) due to timeout, nothing interesting in the log to help me debug this. I'll mark it as failing and reland.

try for a whole bunch of OSX variants: https://treeherder.mozilla.org/jobs?repo=try&revision=a84474bd12f98034a15d3c347ec980135c5ba7e3

Backed out as per Andrew's request.
Also please check again the comment no. 8 for more info about the previous backout.

Backout link

This appears to be causing the TSAN issue, possibly the rooting hazard as well:
https://searchfox.org/mozilla-central/rev/90e5ec580b4334c54829d83d79222d0b3ca4c5df/dom/canvas/CanvasRenderingContext2D.cpp#877

Let's see if this covers everything:
https://treeherder.mozilla.org/jobs?repo=try&revision=f42f6b59b4100a4fdb21750577b89e875e5f1228

https://hg.mozilla.org/mozilla-central/rev/b43727bd95d6
https://hg.mozilla.org/mozilla-central/rev/3e808ae032db
https://hg.mozilla.org/mozilla-central/rev/9712b727646f
https://hg.mozilla.org/mozilla-central/rev/428fb86f9ff1

The docs for this can be tracked in https://github.com/mdn/content/issues/18772

Bug fix says "partial implementation". What is implemented in OffscreenCanvasRenderingContext2D - is it exactly the same methods/properties as CanvasRenderingContext2D?