Edge with input method produces random characters

[intellild]

What's the current behavior?

Can be reproduced with
https://www.slatejs.org/#/plain-text

1. detele all text. so we can see the placeholder
2. input whatever with an input method
3. some random characters come out
   
   Windows 10
   Microsoft Edge 42.17134.1.0
   Microsoft EdgeHTML 17.17134

What's the expected behavior?

No random characters.

My work around

in renderPlaceholder:

  if (editor.state.isComposing) {
    if (IS_EDGE && !editor.props.value.document.text) {
      // noop
    } else {
      return;
    }
  }

[ianstormtaylor]

Closing in favor of #2368, if you think this is unrelated let me know and we can reconsider.