Add a pane to render the AST.

src/App.css

@@ -1,5 +1,7 @@
 .App {
   text-align: center;
+  height: 100vh;
+  width: 100%;
 }
 
 .App-logo {

src/App.tsx

@@ -1,14 +1,10 @@
-import React from 'react';
-import logo from './logo.svg';
 import './App.css';
 import Editor from './Editor';
 
 function App() {
   return (
     <div className="App">
-      <header className="App-header">
-        <Editor />
-      </header>
+      <Editor />
     </div>
   );
 }

src/Editor.css

@@ -0,0 +1,16 @@
+.Editor {
+  display: flex;
+  flex-direction: row;
+  height: 100%;
+  width: 100%;
+}
+
+.Editor-textarea {
+  flex: 1;
+  flex-basis: 0;
+}
+
+.Editor-ast {
+  flex: 1;
+  background: green;
+}

src/Editor.tsx

@@ -1,10 +1,15 @@
+import './Editor.css';
+
 function Editor() {
   function handleChange(event: React.ChangeEvent<HTMLTextAreaElement>) {
     console.log(event.target.value);
   }
 
   return (
-    <textarea onChange={handleChange}></textarea>
+    <div className="Editor">
+      <textarea onChange={handleChange} className="Editor-textarea"></textarea>
+      <div className="Editor-ast"></div>
+    </div>
   );
 }