Confirming for GroupBox mentioned in original report. I have not looked at any other widgets yet.
As far as I could tell, this issue was local to GroupBox.
user: Paul Turner <firstname.lastname@example.org>
date: Wed Jun 06 10:14:16 2012 +0100
summary: FIX: Multiple issues in GroupBox related to handling of its content pane and also how child content is added and removed. NB: This does change some behaviour, in that child content is no longer destroyed when removed, but that should NEVER have happened anyway, so is treated as another bug to be fixed