With this workaround in place, desktop wallpaper (on the working display) cannot be set using the ’Appearance’ configuration tool, and with automatic login the ’login screen as background’ effect (the image attached to #1) is there too. This could be Bug #1159430, and as mentioned there, I can work around it using Compiz’ Wallpaper plugin (i.e. setting a wallpaper in Compiz sets it on the external display).

As I explained in the Edit paragraph I added, the ”solution” was more of the problem as I presented it going away (through related bugs getting fixed) than finding a path of steps to take.

I have two Chromium windows open, 0x31fd46b and 0x3072dc8 (this one):

jani@saegusa:~$ xwininfo -root -tree | grep Chromium-browser
0x31d2a67 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 321×216+275+321 +275+321
0x30fb2de ”Chromium”: (”chromium-browser” ”Chromium-browser”) 246×307+1232+305 +1232+305
0x3157fd9 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 246×307+36+124 +36+124
0x31fd391 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 1×1+-100+-100 +-100+-100
0x31b19f6 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 513×132+1011+38 +1011+38
0x30b5ffa ”Chromium”: (”chromium-browser” ”Chromium-browser”) 397×125+4+82 +4+82
0x3092643 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 321×216+791+829 +791+829
0x3030c84 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 306×230+587+441 +587+441
0x3030681 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 462×342+127+133 +127+133
0x3000027 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 200×200+0+0 +0+0
0x3000024 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 200×200+0+0 +0+0
0x3000001 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 10×10+10+10 +10+10
0x31fd46b ”Yleisurheilun Eliittikisat | Tv | Areena | yle.fi – Chromium”: (”chromium-browser” ”Chromium-browser”) 1014×438+2322+227 +2322+227
0x3072dc8 ”Chromium”: (”chromium-browser” ”Chromium-browser”) 1659×50+94+83 +94+83
0x30000c3 ”Report a bug about “wmctrl” package in Ubuntu : Bugs : “wmctrl” package : Ubuntu – Chromium”: (”chromium-browser” ”Chromium-browser”) 1920×1056+0+0 +0+24

But wmctrl only knows about the one:
jani@saegusa:~$ wmctrl -l
0x01c00006 0 saegusa jani@saegusa: ~
0x02200003 0 N/A DNDCollectionWindow
0x02200004 0 N/A launcher
0x02200006 0 N/A panel
0x02200007 0 N/A Dash
0x02200008 0 N/A Hud
0x030000c3 0 saegusa Report a bug about “wmctrl” package in Ubuntu : Bugs : “wmctrl” package : Ubuntu – Chromium
0x03e00003 0 saegusa Rytmilaatikko
0x04200009 0 saegusa Kuvakaappaus
0x04600081 0 saegusa Synaptic-pakettienhallinta

This may be related to the fact that the other window has fallen out of view: it’s not visible on any workspace, but it’s still there as the audio’s still playing. It got out of sight due to another bug (probably in Compiz) which I have yet to report.