title: When does a run_at: document_idle content script run?
body: <p>There are three options for running content scripts:</p>
  
  <ol>
  <li><code>document_start</code> - injected at the start of the <code>&lt;head&gt;</code></li>
  <li><code>document_end</code> - injected right after <code>DOMContentLoaded</code></li>
  <li><code>document_idle</code> - injected when???</li>
  </ol>