xfce4-panel-4.12.0-3.git20150306.fc21.x86_64 - git e550e16 (--enable-gtk3)

$ PANEL_DEBUG=gdb xfce4-panel
xfce4-panel(main): version 4.12.0git-20150306 on gtk+ 2.24.27 (2.24.27), glib 2.42.2 (2.42.2)
xfce4-panel(main): running external plugins with /usr/bin/gdb, logs stored in /tmp
xfce4-panel(module-factory): reading /usr/share/xfce4/panel/plugins
xfce4-panel(module-factory): reading /usr/share/xfce4/panel-plugins
xfce4-panel(application): found window manager after 1 tries
xfce4-panel(base-window): 0x24a60a0: rgba colormap=0x24b02c0, compositing=true
xfce4-panel(base-window): 0x24a60a0: rgba colormap=0x24b02c0, compositing=true
xfce4-panel(display-layout): 0x24a60a0: display=:0.0{comp=true}, screen-0[0x2471000]=[1920,1080] (DVI-I-1=[0,0;1920,1080])
xfce4-panel(positioning): 0x24a60a0: screen=0x2471000, monitors=1, output-name=(null), span-monitors=false, base=0,0
xfce4-panel(positioning): 0x24a60a0: working-area: screen=0x2471000, x=0, y=0, w=1920, h=1080
xfce4-panel(struts): 0x24a60a0: top=39, start_x=0, end_x=1919
...
xfce4-panel(systray): registered manager on screen 0
...
(wrapper-2.0:2247): libxfce4panel-CRITICAL **: xfce-panel-image.c:748 (xfce_panel_image_scale_pixbuf): expression 'GDK_IS_PIXBUF (source)' failed.
xfce4-panel(application): saving /panels/panel-1: ids=false, providers=true