offscreen can't send message to the paint worker

[totaam]

There were some bugs that prevented the offscreen paint code from actually being used: e740303, 988b150
Now that these are fixed, I can enable it from the advanced options on the connect page, but the result is a blank window!

I have added some debug logging and the message seems to be getting lost between:

xpra-html5/html5/js/OffscreenDecodeWorker.js

Line 162 in bbc3fc3

paint_worker.postMessage(

And

xpra-html5/html5/js/PaintWorker.js

Line 146 in bbc3fc3

onmessage = function (e) {

I thought that perhaps the problem was from the refactoring that added:

xpra-html5/html5/js/PaintWorker.js

Line 13 in bbc3fc3

importScripts("./Constants.js");

So I commented that out and replaced with const DEFAULT_BOX_COLORS = {}; for simplicity.
I also tried removing most attributes in the postMessage call.
Nothing helps!
The javascript console shows this debug message I have added:

sending draw bitmap:rgb24 to paint worker: Worker {onmessage: null, onerror: null}

The onmessage: null is suspicious. But I'm pretty sure that I somehow managed to get some messages across and it was still showing null.