title: css: simulate a "min-top" property
body: <p>I have the following div:</p>
  
  <pre><code>#leftmenu{
  position: fixed;
  top: 50%;
  margin-top: -65x;
  height: 130px;
  }
  </code></pre>
  
  <p>This div is placed vertically centered on the page and it is fixed, the webpage is very tall but the div is always visible in the center of the screen. With these properties, if I resize the window the menu moves because it has to stay at 50% of the current window size. What I want is:<br>
  1) when I load the page, depending on the screen resolution, the div must place 50% top (ok with my code)<br>
  2) if I resize the window, it must stay fixed.
  I hoped there was a property like "min-top" or something like this to force the top level, but it doesn't exist.
  Can you help me?</p>