title: Enabling depth test causes nothing at all to appear
body: <p>I have a depth buffer and am rendering a cone on the screen. It works, but then I realized I had forgotten the standard enabling of depth testing, and when I turned it on, now nothing appears at all.</p>
  
  <p>I only have this one object, a cone, in my scene, so nothing else is in front of it.</p>
  
  <p>The depth buffer is setup fine:</p>
  
  <pre><code>glFramebufferRenderbuffer(GL_FRAMEBUFFER,
                                GL_DEPTH_ATTACHMENT,
                                GL_RENDERBUFFER,
                                m_depthRenderbuffer);
  </code></pre>
  
  <p>The I bind the render buffer:</p>
  
  <pre><code>glBindRenderbuffer(GL_RENDERBUFFER, m_renderbuffer);
  </code></pre>
  
  <p>Then I render the cone:</p>
  
  <p><img src="https://i.stack.imgur.com/jP8I9.png" alt="enter image description here"></p>
  
  <p>(My experiments with this messy cone are the subject of another question I shall ask shortly).</p>
  
  <p>But, as soon as I add this next line after creating the depth buffer and binding the render buffer:</p>
  
  <p><code>glEnable(GL_DEPTH_TEST)</code></p>
  
  <p>...then my cone magically disappears. There is nothing, not even a little dot, on the screen.</p>
  
  <p>Why would that be?</p>