The prettiest way to do this may be to actually wrap it in a dummy xml element like:
```
protected def columns: Seq[Node] = {
  // Use dummy element to wrap the list of columns.
  <dummy>
    <th>Stage Id</th>
    {if (isFairScheduler) {...}
    ...
  </dummy>.child
}
```

The problem is that the XML DSL is so evil that I think we need to either go all the way or avoid it altogether (and we're in too deep to easily avoid it now). All these braces just bring up questions that are better left unasked. For instance, `Seq{if (isFairScheduler) {<th>Pool Name</th>} else {}}` actually returns either a Seq(Node) or a Seq(Unit), which makes virtually no sense, but results in the Seq[Any] you're currently returning.