I believe it’s because the IconManager has been configured to show icons, but the app doesn’t have a mini icon.
Depends on if the windows provide icons themselves or you jave set icons with Style lines. It might help if you provided more information, like your IconMan config.
Hmm, if ‘IconButton flat’ doesn’t give the desied effect I don’t think it’s possible, or it’s pretty obscure anyway…
I think these indicators are useful, but you may try one or another trick to get rid of them.
You don’t seem to use mini icons, so this should work for you:
Style * MiniIcon 1x1-transparent.xpm *FvwmIconMan: DrawIcons always
You may also set sh and hi colors of colorsets 60-65 to their bg color, but this only may work with plain background, not gradient.
Also feel free to implement a new case, like “DrawIcons empty”, should not be very hard.