To run Attachment 9110463 [details], pref gfx.offscreencanvas.enabled:true is necessary, since offscreen canvas is not enabled by default. With it, I confirmed that the event was not raised.
Bug 1598237 Comment 2 Edit History
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To run Attachment 9110463 [details], pref gfx.offscreencanvas.enabled:true is necessary, since offscreen canvas is not enabled by default. By enabling the offscreencanvas, unmute event was raised once.