Hiding combobox decorations doesn’t make them transparent in Windows XP theme

I was trying out the version 2.57 new feature to hide the decorations of comboboxes this morning. It looks like it doesn’t work properly when you’re using the Windows XP theme – the background isn’t transparent, it’s just a white rectangle. All of the other themes work properly though, they all make the decoration invisible as advertised.

It should be pretty easy to spot if you download Xavier’s example plot and switch the theme to Windows XP. Also of note, my computer is running Windows XP, I’m not sure if that matters any.