I would like to give this one a bump. According to everything I read on the JUCE forum, the popup menu is by default opaque, unless you set PopupMenu::backgroundColourId to a colour with an alpha < 1.0.
I've been through all the HISE popup look and feel functions I could find and set this property to transparentWhite but still the combo boxes show a white background.
@Christoph-Hart Any ideas?