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.
Using xprop instead of transset-df has the same effect:
xprop -id 0x02200015 -format _NET_WM_WINDOW_OPACITY 32c -set _NET_WM_WINDOW_OPACITY 3221225471
xprop -id 0x02200015 -remove _NET_WM_WINDOW_OPACITY
You mention this used to work in the past - do you have a specific version in which this still worked?
I tried some commit in the past and it has affect on mod_xrandr or smth like this because I get a lot of strings with errors in xmessage with my actual config. It's something ~year ago, like bfe8b3125861992b792841ae0c337535b53b62d2. But for my current arch rolling I could't compile it even with git submodule update --recursive.
As I say manual setting didn't affect on window.
[~] >> xprop -id 0x03800080 -format _NET_WM_WINDOW_OPACITY 32c -set _NET_WM_WINDOW_OPACITY 3221225471
[~] >> xprop |& ag -i _NET_WM_WIN
_NET_WM_WINDOW_OPACITY(CARDINAL) = 3221225471
_NET_WM_WINDOW_TYPE(ATOM) = _NET_WM_WINDOW_TYPE_NORMAL
May you comment this code? http://sprunge.us/eDHH
Current window return WScreen value instead of current win. Is it right behaviour? Maybe transset look for window "WScreen"? So, maybe children of WScreen get value ?
Interesting: I can confirm the xprop commands I provided above don't seem to have any effect anymore, and that indeed 'transset' updates _NET_WM_WINDOW_OPACITY for the root WScreen