@discussion-width: 810px;

.discussion2 {
  width: 810px;
  margin: 90px auto 0;
  z-index: 0;
  position: relative;
}

@discussion-width: 810px;

.discussion2 {
  width: 810px;
  margin: 0 auto;
  padding: 90px 0 0;
  z-index: 0;
  position: relative;
}

Use padding instead of margin, to make sure the page stays 100% height
Use padding instead of margin, to make sure the page stays 100% height
