There is no correct work of transset-df, so it seems that transparent attribute set to all windows (include root?) instead of only one. But dmenu or dunst with opacity setting works well. Manual setting of _NET_WM_WINDOW_OPACITY didn't get affect on window at all.
I tried compton for it.
Once I tried to find a bug, but not all old versions did compile now with new lua libs(?), so I can't bisect bad commit.