I bisected it. It seems the composite operation is being thrown out by pixman due to some problem with the source surface.
The backtrace when that happens is this:


(gdb) bt
#0  do_composite (op=<value optimised out>, src=<value optimised out>, mask=0x0, dest=<value optimised out>, src_x=<value optimised out>, src_y=<value optimised out>, mask_x=352, 
    mask_y=0, dest_x=352, dest_y=0, width=20, height=61) at pixman.c:882
#1  pixman_image_composite32 (op=<value optimised out>, src=<value optimised out>, mask=0x0, dest=<value optimised out>, src_x=<value optimised out>, src_y=<value optimised out>, 
    mask_x=352, mask_y=0, dest_x=352, dest_y=0, width=20, height=61) at pixman.c:983
#2  0x00007ffff60d5ad3 in _composite_boxes (dst=0x748b10, op=<value optimised out>, src=<value optimised out>, boxes=<value optimised out>, antialias=<value optimised out>, 
    extents=<value optimised out>, clip=0x0) at /build/buildd/cairo-1.10.0/src/cairo-image-surface.c:2976
#3  _clip_and_composite_boxes (dst=0x748b10, op=<value optimised out>, src=<value optimised out>, boxes=<value optimised out>, antialias=<value optimised out>, 
    extents=<value optimised out>, clip=0x0) at /build/buildd/cairo-1.10.0/src/cairo-image-surface.c:3015
#4  0x00007ffff60d685b in _cairo_image_surface_paint (abstract_surface=0x748b10, op=CAIRO_OPERATOR_OVER, source=0x7fffffffd3e0, clip=0x0)
    at /build/buildd/cairo-1.10.0/src/cairo-image-surface.c:3269
#5  0x00007ffff60f4657 in _cairo_surface_paint (surface=0x748b10, op=CAIRO_OPERATOR_OVER, source=0x7fffffffd3e0, clip=0x7fffffffd4e0)
    at /build/buildd/cairo-1.10.0/src/cairo-surface.c:2022
#6  0x00007ffff60cc222 in _cairo_gstate_paint (gstate=0x7ffff63628d0) at /build/buildd/cairo-1.10.0/src/cairo-gstate.c:1049
#7  0x00007ffff60c35d4 in cairo_paint (cr=0x7ffff63628a0) at /build/buildd/cairo-1.10.0/src/cairo.c:2227
#8  0x00007ffff60c36c8 in cairo_paint_with_alpha (cr=0x7ffff63628a0, alpha=-4294966924) at /build/buildd/cairo-1.10.0/src/cairo.c:2255
#9  0x00000000004215c8 in _pad_paint ()
#10 0x00000000004219e1 in tile_paint_with_padding ()
#11 0x0000000000412db1 in _refresh_background ()
#12 0x00000000004138a9 in bubble_recalc_size ()
#13 0x000000000041d5e9 in stack_notify_handler ()
#14 0x000000000041c126 in dbus_glib_marshal_stack_VOID__STRING_UINT_STRING_STRING_STRING_BOXED_BOXED_INT_POINTER ()
#15 0x00007ffff7707fef in invoke_object_method (object=<value optimised out>, object_info=<value optimised out>, method=0x429ca0, connection=<value optimised out>, 
    message=<value optimised out>) at dbus-gobject.c:1724
#16 0x00007ffff77084bd in object_registration_message (connection=<value optimised out>, message=<value optimised out>, user_data=<value optimised out>) at dbus-gobject.c:1944
#17 0x00007ffff74d5dde in ?? () from /lib/libdbus-1.so.3
#18 0x00007ffff74c948c in dbus_connection_dispatch () from /lib/libdbus-1.so.3
#19 0x00007ffff7704a45 in message_queue_dispatch (source=<value optimised out>, callback=<value optimised out>, user_data=<value optimised out>) at dbus-gmain.c:101
#20 0x00007ffff576f342 in g_main_dispatch (context=0x66f0c0) at /build/buildd/glib2.0-2.26.1/glib/gmain.c:2149
#21 g_main_context_dispatch (context=0x66f0c0) at /build/buildd/glib2.0-2.26.1/glib/gmain.c:2702
#22 0x00007ffff57732a8 in g_main_context_iterate (context=0x66f0c0, block=<value optimised out>, dispatch=<value optimised out>, self=<value optimised out>)
    at /build/buildd/glib2.0-2.26.1/glib/gmain.c:2780
#23 0x00007ffff57737b5 in g_main_loop_run (loop=0x6dd9c0) at /build/buildd/glib2.0-2.26.1/glib/gmain.c:2988
#24 0x00007ffff6d913e7 in IA__gtk_main () at /build/buildd/gtk+2.0-2.22.0/gtk/gtkmain.c:1237
#25 0x000000000041b60d in main ()