fix void node cursor navigation in firefox

lib/components/void.js

@@ -5,6 +5,7 @@ import OffsetKey from '../utils/offset-key'
 import React from 'react'
 import ReactDOM from 'react-dom'
 import keycode from 'keycode'
+import { IS_FIREFOX } from '../utils/environment'
 
 /**
  * Void.
@@ -109,15 +110,26 @@ class Void extends React.Component {
    */
 
   renderSpacer = () => {
-    const style = {
-      position: 'absolute',
-      top: '0px',
-      left: '-9999px',
-      textIndent: '-9999px'
-    }
+    // COMPAT: In Firefox, if the <span> is positioned absolutely, it won't
+    // receive the cursor properly when navigating via arrow keys.
+    const style = IS_FIREFOX
+      ? {
+          pointerEvents: 'none',
+          width: '0px',
+          height: '0px',
+          lineHeight: '0px',
+          visibility: 'hidden'
+        }
+      : {
+          pointerEvents: 'none',
+          position: 'absolute',
+          top: '0px',
+          left: '-9999px',
+          textIndent: '-9999px'
+        }
 
     return (
-      <span style={style}>{this.renderLeaf()}</span>
+      <span style={style}><span>{this.renderLeaf()}</span></span>
     )
   }