title: Listen to RichTextBox.ZoomFactor has changed event
body: <p>I cannot determine why the ZoomFactor of my Winforms RichTextBox is getting reset to 1.</p>
  <p>I checked all calls to <code>Clear()</code> and when setting the <code>Text</code> property and I am aware of the <em>Set-ZoomFactor-twice</em>-bug (<a href="https://stackoverflow.com/questions/64929389/need-to-set-richtextbox-zoomfactor-twice">Need to set RichTextBox ZoomFactor twice?</a>).</p>
  <p>Now I wonder whether there is a way to debug into some <em>ZoomFactor-has-changed</em>-event in order to determine why and how the ZoomFactor is changed automatically.</p>
  <p>Any help is highly appreciated.</p>
  <p><strong>Edit 1</strong></p>
  <p>I can reproduce the reset as follows (at least in my application):</p>
  <ul>
  <li>Click into empty RichTextBox</li>
  <li>Increase zoom using Ctrl and mouse wheel</li>
  <li>Click somewhere else</li>
  <li>Click into the RichtTextBox again</li>
  </ul>
  <p>This suggests that the ZoomFactor is automatically reset if the Text property is empty and the RichTextBox becomes focused, not just when calling <code>Clear()</code>. Could this be the reason?</p>
  <p><strong>Edit 2</strong></p>
  <p>I could <em>not</em> reproduce the effect using a new Winforms example project and the above procedure. This means, I have to look for some side effects in my application.</p>
  <p><strong>Edit 3</strong></p>
  <p>The ZoomFactor has been reset due to an overlooked <code>Text = &lt;empty string&gt;</code>. It is working now.</p>