[w3c/editing] How can I send the return value after I bind an copy/cut event after propagation stoped (#157)

When I bind event like that code
```javascript
document.addEventListener('copy', (e) => {
      e.preventDefault()
      e.stopPropagation()
      this.handle(e)
      e.returnValue = true
      return true
    }, false)
```
I wish the returnValue could be used for execCommand('copy') validator ,but now it's always false.
I can't get the real state clipboard stay in.
What should I do?

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