Detached OffscreenCanvas is underspecified about html HOT 2 OPEN

Setting width/height or attempting to read the bitmap of a detached OffscreenCanvas are programming errors IMHO. I am pretty sure throwing an InvalidState exception is the right thing to do in almost all cases.

For the placeholder situation, could you clarify what the problematic use case is? Setting the width or height on a placeholder canvas (i.e. after calling transferControlToOffscreen) is supposed to be a no-op. Spec'ed here: https://html.spec.whatwg.org/multipage/canvas.html#concept-canvas-placeholder
As far as the spec is concerned, the placeholder canvas does not store a reference to the OffscreenCanvas that controls it (though implementations may have an internal reference that has no impact on API behavior). The only reference that is prescribed by the spec is in the other direction: the OffscreenCanvas has an optional weak reference to a placeholder canvas. The canvas element knows that it is being controlled by an offscreen canvas via its context mode, which may be set to placeholder, but it does not hold a reference to the actual OffscreenCanvas. There is no data flow from the placeholder to the OffscreenCanvas; data only flows in the other direction. This unidirectionality makes it trivial to ensure that the spec is free of signals that would be vulnerable to deadlocks or feedback loops.

from html.

Setting width/height or attempting to read the bitmap of a detached OffscreenCanvas are programming errors IMHO. I am pretty sure throwing an InvalidState exception is the right thing to do in almost all cases.

Agreed, I'll take it as a Chrome position and will try to prepare a PR to that effect when I get time.

For the placeholder situation, could you clarify what the problematic use case is?

I was only talking about the relation OffscreenCanvas => placeholder<canvas>. I simply noticed that the OffscreenCanvas's placeholder canvas element isn't unset in the transfer steps and since the width and height setters aren't defined (yet) for when context mode is detached what happens to the placeholder canvas is also unspecced.

But I now see that even the context mode: none has some small interop issues with placeholder canvases, and the specs seem unclear here too. See this fiddle where we can see that Safari does resize the placeholder canvas even when there is no context on the OffscreenCanvas, while Firefox will wait until the context is created and Chrome until there is actual drawings on the context. All the specs say is that commit() was supposed to be updating the placeholder size, which is going to be replaced by "update the rendering".

from html.