ToolBuilder: list in panel appears grey

I was hacking on the ClassBrowser (so
Browser>>openOnClassWithEditString:), working out how to make the
instance/?/class buttons have constant height. So I did this, which goes
inside a suitable frame (0.5@0.0 corner: 1@max):

buildClassBrowserMessageListWith: builder
        | list switches top |
        top := builder pluggablePanelSpec new.
        top children: OrderedCollection new.
       
        switches := self buildSwitchesWith: builder.
        switches frame: (LayoutFrame new
                leftFraction: 0 offset: 0;
                topFraction: 0 offset: 0;
                rightFraction: 1 offset: 0;
                bottomFraction: 0 offset: self buttonHeight).
               
        list := self buildMessageListWith: builder.
        list frame: (LayoutFrame new
                leftFraction: 0 offset: 0;
                topFraction: 0 offset: self buttonHeight;
                rightFraction: 1 offset: 0;
                bottomFraction: 1 offset: 0).
               
        top children add: switches.
        top children add: list.
       
        ^top.

OK, the method name's not great. Anyway. I expected list above to have a
white background, but it doesn't. It has a grey background, like the
underlying PluggablePanel. See attachment.

So, to my question: what have I done wrong? Why's the message list grey
while the category list is white?

frank