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  function modifyGetContext(scope, env) {
    let dispatchEvent = EventTarget.prototype.dispatchEvent;
    let { Event } = scope;
    for (let canvas of ["HTMLCanvasElement", "OffscreenCanvas"]) {
      if (!(canvas in scope)) continue;

      const getContext = scope[canvas].prototype.getContext;

      const handler = cloneInto({
        apply: function(targetObj, thisArg, argumentsList) {
          if (thisArg instanceof window[canvas] && /webgl/i.test(argumentsList[0])) {
            let target = canvas === "HTMLCanvasElement" && document.contains(thisArg) ? thisArg : scope;
            env.port.postMessage("webgl", target);
            return null;
          }
          return getContext.call(thisArg, ...argumentsList);
        }
      }, scope, {cloneFunctions: true});

      const proxy = new scope.Proxy(getContext, handler);
      scope[canvas].prototype.getContext = proxy;
    }
  }