- mixer debug:
minime@beaver:~/mixerdebug $ PANEL_DEBUG=mixer xfce4-panel 
xfce4-panel(main): version 4.10.0 on gtk+ 2.24.6 (2.24.6), glib 2.28.8 (2.28.8)
xfce4-panel(module-factory): reading /usr/local/share/xfce4/panel/plugins
xfce4-panel(module-factory): reading /usr/local/share/xfce4/panel-plugins
xfce4-panel(application): found window manager after 1 tries
xfce4-panel(base-window): 0x8089e4060: rgba colormap=0x808847b30, compositing=false
xfce4-panel(base-window): 0x8089e4060: rgba colormap=0x808847b30, compositing=false
xfce4-panel(display-layout): 0x8089e4060: display=:0.0{comp=true}, screen-0[0x808985600]=[1920,1200] (DVI-I-3=[0,0;1920,1200])
xfce4-panel(positioning): 0x8089e4060: screen=0x808985600, monitors=1, output-name=(null), span-monitors=false, base=0,0
xfce4-panel(positioning): 0x8089e4060: working-area: screen=0x808985600, x=0, y=0, w=1920, h=1200
xfce4-panel(struts): 0x8089e4060: top=31, start_x=0, end_x=1919
xfce4-panel(module): new item (type=object-type, name=showdesktop, id=1)
xfce4-panel(module): new item (type=object-type, name=pager, id=2)
xfce4-panel(module): new item (type=object-type, name=tasklist, id=15)
xfce4-panel(module): new item (type=object-type, name=separator, id=16)
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/4
xfce4-panel(module): new item (type=external-wrapper, name=fsguard, id=4)
xfce4-panel(external): fsguard-4: child spawned; pid=38368, argc=8
xfce4-panel(module): new item (type=object-type, name=launcher, id=9)
xfce4-panel(module): new item (type=object-type, name=launcher, id=5)
xfce4-panel(module): new item (type=object-type, name=launcher, id=13)
xfce4-panel(module): new item (type=object-type, name=launcher, id=6)
xfce4-panel(module): new item (type=object-type, name=launcher, id=8)
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/3
xfce4-panel(module): new item (type=external-wrapper, name=mixer, id=3)
xfce4-panel(external): mixer-3: child spawned; pid=38369, argc=8
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/11
xfce4-panel(module): new item (type=external-wrapper, name=systray, id=11)
xfce4-panel(external): systray-11: child spawned; pid=38370, argc=8
xfce4-panel(module): new item (type=object-type, name=clock, id=12)
xfce4-panel(external): fsguard-4: child is embedded; 5 properties in queue
xfce4-panel(systray): registered manager on screen 0
xfce4-panel(external): systray-11: child is embedded; 5 properties in queue
xfce4-panel(external): mixer-3: child is embedded; 5 properties in queue
xfce4-panel(external): mixer-3: child is unembedded
xfce4-panel(external): mixer-3: child exited with status 9
xfce4-panel-Message: Plugin mixer-3 has been automatically restarted after crash.
xfce4-panel(external): mixer-3: scheduled a respawn of the child
xfce4-panel(external): mixer-3: child spawned; pid=38375, argc=8
xfce4-panel(external): mixer-3: child is embedded; 5 properties in queue
xfce4-panel(external): mixer-3: child is unembedded
xfce4-panel(external): mixer-3: child exited with status 9
xfce4-panel(external): mixer-3: scheduled a respawn of the child
xfce4-panel(external): mixer-3: child spawned; pid=38378, argc=8
xfce4-panel(external): mixer-3: child is embedded; 5 properties in queue
- gdb debug (no file in /tmp/ is created, panel starts but mixer icon is not loaded)
minime@beaver:~/mixerdebug $ PANEL_DEBUG=gdb xfce4-panel
xfce4-panel(main): version 4.10.0 on gtk+ 2.24.6 (2.24.6), glib 2.28.8 (2.28.8)
xfce4-panel(main): running external plugins with /usr/bin/gdb, logs stored in /tmp
xfce4-panel(module-factory): reading /usr/local/share/xfce4/panel/plugins
xfce4-panel(module-factory): reading /usr/local/share/xfce4/panel-plugins
xfce4-panel(application): found window manager after 1 tries
xfce4-panel(base-window): 0x8089a0060: rgba colormap=0x808847b30, compositing=false
xfce4-panel(base-window): 0x8089a0060: rgba colormap=0x808847b30, compositing=false
xfce4-panel(display-layout): 0x8089a0060: display=:0.0{comp=true}, screen-0[0x808886600]=[1920,1200] (DVI-I-3=[0,0;1920,1200])
xfce4-panel(positioning): 0x8089a0060: screen=0x808886600, monitors=1, output-name=(null), span-monitors=false, base=0,0
xfce4-panel(positioning): 0x8089a0060: working-area: screen=0x808886600, x=0, y=0, w=1920, h=1200
xfce4-panel(struts): 0x8089a0060: top=31, start_x=0, end_x=1919
xfce4-panel(module): new item (type=object-type, name=showdesktop, id=1)
xfce4-panel(module): new item (type=object-type, name=pager, id=2)
xfce4-panel(module): new item (type=object-type, name=tasklist, id=15)
xfce4-panel(module): new item (type=object-type, name=separator, id=16)
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/4
xfce4-panel(module): new item (type=external-wrapper, name=fsguard, id=4)
xfce4-panel(external): fsguard-4: child spawned; pid=84212, argc=25
xfce4-panel(module): new item (type=object-type, name=launcher, id=9)
xfce4-panel(module): new item (type=object-type, name=launcher, id=5)
xfce4-panel(module): new item (type=object-type, name=launcher, id=13)
xfce4-panel(module): new item (type=object-type, name=launcher, id=6)
xfce4-panel(module): new item (type=object-type, name=launcher, id=8)
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/3
xfce4-panel(module): new item (type=external-wrapper, name=mixer, id=3)
xfce4-panel(external): mixer-3: child spawned; pid=84213, argc=25
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/11
xfce4-panel(module): new item (type=external-wrapper, name=systray, id=11)
xfce4-panel(external): systray-11: child spawned; pid=84214, argc=25
xfce4-panel(module): new item (type=object-type, name=clock, id=12)
xfce4-panel(external): fsguard-4: child exited with status 0
xfce4-panel(external): mixer-3: child exited with status 0
xfce4-panel(external): systray-11: child exited with status 0
xfce4-panel(external): fsguard-4: plugin unrealized; quiting child
xfce4-panel(external): mixer-3: plugin unrealized; quiting child
xfce4-panel(external): systray-11: plugin unrealized; quiting child
xfce4-panel(application): finalized

- display-layout debug
minime@beaver:~/mixerdebug $ PANEL_DEBUG=display-layout xfce4-panel
xfce4-panel(main): version 4.10.0 on gtk+ 2.24.6 (2.24.6), glib 2.28.8 (2.28.8)
xfce4-panel(module-factory): reading /usr/local/share/xfce4/panel/plugins
xfce4-panel(module-factory): reading /usr/local/share/xfce4/panel-plugins
xfce4-panel(application): found window manager after 1 tries
xfce4-panel(base-window): 0x8089e4060: rgba colormap=0x808847b30, compositing=false
xfce4-panel(base-window): 0x8089e4060: rgba colormap=0x808847b30, compositing=false
xfce4-panel(display-layout): 0x8089e4060: display=:0.0{comp=true}, screen-0[0x808985600]=[1920,1200]{comp=false, sys=0x808846840:0x808809000, rgba=0x808847b30:0x80880e120} (DVI-I-3=[0,0;1920,1200])
xfce4-panel(positioning): 0x8089e4060: screen=0x808985600, monitors=1, output-name=(null), span-monitors=false, base=0,0
xfce4-panel(positioning): 0x8089e4060: working-area: screen=0x808985600, x=0, y=0, w=1920, h=1200
xfce4-panel(struts): 0x8089e4060: top=31, start_x=0, end_x=1919
xfce4-panel(module): new item (type=object-type, name=showdesktop, id=1)
xfce4-panel(module): new item (type=object-type, name=pager, id=2)
xfce4-panel(module): new item (type=object-type, name=tasklist, id=15)
xfce4-panel(module): new item (type=object-type, name=separator, id=16)
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/4
xfce4-panel(module): new item (type=external-wrapper, name=fsguard, id=4)
xfce4-panel(external): fsguard-4: child spawned; pid=84234, argc=8
xfce4-panel(module): new item (type=object-type, name=launcher, id=9)
xfce4-panel(module): new item (type=object-type, name=launcher, id=5)
xfce4-panel(module): new item (type=object-type, name=launcher, id=13)
xfce4-panel(module): new item (type=object-type, name=launcher, id=6)
xfce4-panel(module): new item (type=object-type, name=launcher, id=8)
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/3
xfce4-panel(module): new item (type=external-wrapper, name=mixer, id=3)
xfce4-panel(external): mixer-3: child spawned; pid=84235, argc=8
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/11
xfce4-panel(module): new item (type=external-wrapper, name=systray, id=11)
xfce4-panel(external): systray-11: child spawned; pid=84236, argc=8
xfce4-panel(module): new item (type=object-type, name=clock, id=12)
xfce4-panel(external): fsguard-4: child is embedded; 5 properties in queue
xfce4-panel(systray): registered manager on screen 0
xfce4-panel(external): systray-11: child is embedded; 5 properties in queue
xfce4-panel(external): mixer-3: child is embedded; 5 properties in queue
xfce4-panel(external): mixer-3: child is unembedded
xfce4-panel(external): mixer-3: child exited with status 9
xfce4-panel-Message: Plugin mixer-3 has been automatically restarted after crash.
xfce4-panel(external): mixer-3: scheduled a respawn of the child
xfce4-panel(external): mixer-3: child spawned; pid=84239, argc=8
xfce4-panel(external): mixer-3: child is embedded; 5 properties in queue
xfce4-panel(external): mixer-3: child is unembedded
xfce4-panel(external): mixer-3: child exited with status 15
xfce4-panel(external): mixer-3: scheduled a respawn of the child
xfce4-panel(external): mixer-3: child spawned; pid=84242, argc=8
xfce4-panel(external): mixer-3: child is embedded; 5 properties in queue
xfce4-panel(external): fsguard-4: plugin unrealized; quiting child
xfce4-panel(external): mixer-3: plugin unrealized; quiting child
xfce4-panel(external): systray-11: plugin unrealized; quiting child
xfce4-panel(systray): unregistered manager
xfce4-panel(application): finalized