title: Material UI - Button doesn't work over another component
body: <p>The button has the following style: </p>
  
  <pre><code>floatButton: {
      position: 'fixed',
      bottom: theme.spacing.unit * 2,
      right: theme.spacing.unit * 2
  }
  </code></pre>
  
  <p>And the button component looks like:</p>
  
  <pre><code>&lt;Button
    className={classes.floatButton}
    variant="fab"
    color="secondary"
    onClick={this.handleHideElement}
  &gt;
    &lt;AddIcon /&gt;
  &lt;/Button&gt;
  </code></pre>
  
  <p>The problem happens when the browser windows is small and a I can't click the button:</p>
  
  <p><a href="https://i.stack.imgur.com/PhxvU.png" rel="nofollow noreferrer">enter image description here</a></p>
  
  <p>For some reason the button is a little transparent and the onClick function doesn't work.</p>
  
  <p>How can i resolve this?</p>
  
  <p>Thank you.</p>