Fixed XenForo.stackAlert doesn't return its html holder like XenForo.alert

[refael]

I guess it could be classified as suggestion, but it has a point of inconsistency.

XenForo.stackAlert doesn't return its html holder, which means it cannot be modified (not easily anyway).
It's basically should return $message, just like XenForo.alert returns $overlayHtml.

 

[Mike]

Not exactly a bug, but there's no BC break to return $message since it doesn't return anything right now.