message: Add some comments next to some confusing lines of code.
diff: @@ -69,6 +69,7 @@ class Inner extends React.PureComponent {
  }
  }
  +// Keep track of seen events so we only animate new ones.
  const seen = new Set();
  const timeout = {enter: 150};
  @@ -96,6 +97,7 @@ class SysEvent extends React.Component {
  let shouldAnimate;
  const key = metadata.uid;
  + // Only animate events if they're at the start of the list (first 6) and we haven't seen them before.
  if (!seen.has(key) && index < 6) {
  seen.add(key);
  shouldAnimate = true;
  