Fix floating caret position incorrect while scrolling overlay (#4024)

Author: jihchi

packages/react-error-overlay/src/components/CodeBlock.js

@@ -10,6 +10,7 @@ import React from 'react';
 import { redTransparent, yellowTransparent } from '../styles';
 
 const _preStyle = {
+  position: 'relative',
   display: 'block',
   padding: '0.5em',
   marginTop: '0.5em',