I just built everything from Subversion (revision 120). In my BorderLayout ContentPanels the title bar buttons sit on top of each other rather than floating horizontally. When I add a float: right rule to the .ylayout-tools-button selector all seems to work again.
Curiously, it looks like the 0.40 playpen CSS has this rule (http://yui-ext.com/playpen/yui-ext.0...css/layout.css). Any idea what might have happened?
float: right; /* missing from latest (rev 120) of layout.css