log with `PANEL_DEBUG=1 xfce4-panel`

panel log

xfce4-panel(main): version 4.10.0 on gtk+ 2.24.17 (2.24.15), glib 2.36.1 (2.34.3)
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): 0x186e180: rgba colormap=0x1878230, compositing=true
xfce4-panel(base-window): 0x186e180: rgba colormap=0x1878230, compositing=true
xfce4-panel(display-layout): 0x186e180: display=:0.0{comp=true}, screen-0[0x1808400]=[1366,768] (LVDS1=[0,0;1366,768])
xfce4-panel(positioning): 0x186e180: screen=0x1808400, monitors=1, output-name=(null), span-monitors=false, base=0,0
xfce4-panel(positioning): 0x186e180: working-area: screen=0x1808400, x=0, y=0, w=1366, h=768
xfce4-panel(struts): 0x186e180: top=31, start_x=0, end_x=1365
xfce4-panel(applicationsmenu): XDG_MENU_PREFIX is set to "xfce-", menu environment is "XFCE"
xfce4-panel(module): new item (type=object-type, name=applicationsmenu, id=1)
xfce4-panel(module): new item (type=object-type, name=launcher, id=19)
xfce4-panel(module): new item (type=object-type, name=launcher, id=20)
xfce4-panel(module): new item (type=object-type, name=launcher, id=9)
xfce4-panel(module): new item (type=object-type, name=launcher, id=4)
xfce4-panel(module): new item (type=object-type, name=launcher, id=14)
xfce4-panel(module): new item (type=object-type, name=launcher, id=22)
xfce4-panel(module): new item (type=object-type, name=launcher, id=26)
xfce4-panel(module): new item (type=object-type, name=launcher, id=25)
xfce4-panel(module): new item (type=object-type, name=launcher, id=29)
xfce4-panel(module): new item (type=object-type, name=launcher, id=11)
xfce4-panel(module): new item (type=object-type, name=launcher, id=2)
xfce4-panel(module): new item (type=object-type, name=launcher, id=13)
xfce4-panel(module): new item (type=object-type, name=launcher, id=7)
xfce4-panel(module): new item (type=object-type, name=launcher, id=17)
xfce4-panel(module): new item (type=object-type, name=launcher, id=18)
xfce4-panel(module): new item (type=object-type, name=launcher, id=23)
xfce4-panel(module): new item (type=object-type, name=separator, id=16)
xfce4-panel(module): new item (type=object-type, name=clock, id=5)
xfce4-panel(module): new item (type=object-type, name=launcher, id=31)
xfce4-panel(module): new item (type=object-type, name=pager, id=15)
xfce4-panel(base-window): 0x186e400: rgba colormap=0x1878230, compositing=true
xfce4-panel(base-window): 0x186e400: rgba colormap=0x1878230, compositing=true
xfce4-panel(display-layout): 0x186e400: display=:0.0{comp=true}, screen-0[0x1808400]=[1366,768] (LVDS1=[0,0;1366,768])
xfce4-panel(positioning): 0x186e400: screen=0x1808400, monitors=1, output-name=(null), span-monitors=false, base=683,754
xfce4-panel(positioning): 0x186e400: working-area: screen=0x1808400, x=0, y=0, w=1366, h=768
xfce4-panel(struts): 0x186e400: bottom=31, start_x=0, end_x=1365
xfce4-panel(module): new item (type=object-type, name=tasklist, id=3)
xfce4-panel(module): new item (type=object-type, name=separator, id=8)
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/6
xfce4-panel(module): new item (type=external-wrapper, name=systray, id=6)
xfce4-panel(external): systray-6: child spawned; pid=5003, argc=8
xfce4-panel(module): new item (type=external-46, name=mailwatch, id=10)
xfce4-panel(external): mailwatch-10: child spawned; pid=5010, argc=8
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/21
xfce4-panel(module): new item (type=external-wrapper, name=fsguard, id=21)
xfce4-panel(xfce4-mailwatch-plugin): compiled against libxfce4panel 4.10.0
xfce4-panel(external): fsguard-21: child spawned; pid=5012, argc=8
xfce4-panel(module): new item (type=external-46, name=netload, id=12)
xfce4-panel(external): netload-12: child spawned; pid=5013, argc=8
xfce4-panel(external): register dbus path /org/xfce/Panel/Wrapper/27
xfce4-panel(module): new item (type=external-wrapper, name=cpugraph, id=27)
xfce4-panel(xfce4-netload-plugin): compiled against libxfce4panel 4.10.0
xfce4-panel(external): cpugraph-27: child spawned; pid=5014, argc=8
xfce4-panel(systray): registered manager on screen 0
xfce4-panel(mailwatch): calling plugin construct function
xfce4-panel(netload): calling plugin construct function
xfce4-panel(mailwatch): send provider signal 13
xfce4-panel(netload): send provider signal 13
xfce4-panel(mailwatch): send provider signal 14
xfce4-panel(external): mailwatch-10: child is embedded; 6 properties in queue
xfce4-panel(external): systray-6: child is embedded; 6 properties in queue
xfce4-panel(external): fsguard-21: child is embedded; 6 properties in queue
xfce4-panel(external): netload-12: child is embedded; 6 properties in queue
xfce4-panel(external): cpugraph-27: child is embedded; 6 properties in queue

(process:5433): GLib-CRITICAL **: g_slice_set_config: assertion `sys_page_size == 0' failed

(process:9315): GLib-CRITICAL **: g_slice_set_config: assertion `sys_page_size == 0' failed
Created new window in existing browser session.

(process:10841): GLib-CRITICAL **: g_slice_set_config: assertion `sys_page_size == 0' failed
SendMessage (69206083, 0x101f, (nil), (nil))
SendMessage (0, 0x1203, (nil), 0x7fffa1604180)
SendMessage (0, 0x1204, (nil), 0x7fffa1604180)
SendMessage (0, 0x1203, 0x1, 0x7fffa1604180)
SendMessage (0, 0x1204, 0x1, 0x7fffa1604180)
SendMessage (0, 0x1203, 0x2, 0x7fffa1604180)
SendMessage (0, 0x1204, 0x2, 0x7fffa1604180)
SendMessage (0, 0x1203, 0x3, 0x7fffa1604180)
SendMessage (0, 0x1204, 0x3, 0x7fffa1604180)
SendMessage (0, 0x1203, 0x4, 0x7fffa1604180)
SendMessage (0, 0x1204, 0x4, 0x7fffa1604180)
SendMessage (69206083, 0x101f, (nil), (nil))
SendMessage (0, 0x1203, (nil), 0x7fffa1604160)
SendMessage (0, 0x1204, (nil), 0x7fffa1604160)
SendMessage (0, 0x1203, 0x1, 0x7fffa1604160)
SendMessage (0, 0x1204, 0x1, 0x7fffa1604160)
SendMessage (0, 0x1203, 0x2, 0x7fffa1604160)
SendMessage (0, 0x1204, 0x2, 0x7fffa1604160)
SendMessage (0, 0x1203, 0x3, 0x7fffa1604160)
SendMessage (0, 0x1204, 0x3, 0x7fffa1604160)
SendMessage (0, 0x1203, 0x4, 0x7fffa1604160)
SendMessage (0, 0x1204, 0x4, 0x7fffa1604160)
SendMessage (69206083, 0x101f, (nil), (nil))
SendMessage (0, 0x1203, (nil), 0x7fffa1605240)
SendMessage (0, 0x1204, (nil), 0x7fffa1605240)
SendMessage (0, 0x1203, 0x1, 0x7fffa1605240)
SendMessage (0, 0x1204, 0x1, 0x7fffa1605240)
SendMessage (0, 0x1203, 0x2, 0x7fffa1605240)
SendMessage (0, 0x1204, 0x2, 0x7fffa1605240)
SendMessage (0, 0x1203, 0x3, 0x7fffa1605240)
SendMessage (0, 0x1204, 0x3, 0x7fffa1605240)
SendMessage (0, 0x1203, 0x4, 0x7fffa1605240)
SendMessage (0, 0x1204, 0x4, 0x7fffa1605240)
SendMessage (69206075, 0x444, 0x1, 0x2ace760)
SendMessage (69206075, 0x444, 0x1, 0x2af3350)
SendMessage (69206075, 0x444, 0x1, 0x2afc690)
SendMessage (69206075, 0x444, 0x1, 0x29e1f10)
SendMessage (69206075, 0x444, 0x1, 0x2b07bf0)
SendMessage (69206075, 0x444, 0x1, 0x2b2c670)
SendMessage (69206075, 0x444, 0x1, 0x2b4dea0)
SendMessage (69206075, 0x444, 0x1, 0x2ab0b10)
SendMessage (69206075, 0x444, 0x1, 0x2b981f0)
SendMessage (69206075, 0x444, 0x1, 0x2b34980)
SendMessage (69206075, 0x444, 0x1, 0x2d4c6c0)

(process:10988): GLib-CRITICAL **: g_slice_set_config: assertion `sys_page_size == 0' failed
Created new window in existing browser session.
Using method `randr'.
xfce4-panel(applicationsmenu): loading from /etc/xdg/menus/xfce-applications.menu
libpng warning: zero length keyword
libpng warning: Empty language field in iTXt chunk
libpng warning: zero length keyword
libpng warning: Empty language field in iTXt chunk
libpng warning: zero length keyword
libpng warning: Empty language field in iTXt chunk
libpng warning: zero length keyword
libpng warning: Empty language field in iTXt chunk
  GimpImage "anonymous": 3950522(6256)
    GimpUndoStack "anonymous": 152(0)
      GimpList "GimpUndo": 72(0)
    GimpUndoStack "anonymous": 928388(0)
      GimpList "GimpUndo": 928308(0)
        GimpDrawableUndo "Bucket Fill": 928212(6944)
    GimpParasiteList "anonymous": 167(0)
    GimpSelection "Selection Mask": 313872(0)
      GimpParasiteList "anonymous": 40(0)
    GimpItemTree "anonymous": 124(0)
      GimpItemStack "GimpVectors": 84(0)
    GimpItemTree "anonymous": 409(0)
      GimpDrawableStack "GimpChannel": 369(0)
    GimpItemTree "anonymous": 928581(0)
      GimpDrawableStack "GimpLayer": 928541(0)
        GimpLayer "Background": 928219(40032)
          GimpParasiteList "anonymous": 40(0)
    GimpGrid "Default Grid": 165(0)
    GimpProjection "anonymous": 1778608(0)

(process:30911): GLib-CRITICAL **: g_slice_set_config: assertion `sys_page_size == 0' failed
Created new window in existing browser session.

(process:32635): GLib-CRITICAL **: g_slice_set_config: assertion `sys_page_size == 0' failed

This looks like something that should go in a proper bugreport on bugs.xfce.org