It appears to be related to Xinerama. I reconfigured X without it (`nvidia-xconfig --twinview --no-xinerama'), and it has stopped crashing.

I also got a backtrace. I know it's not a very good one. I installed several dozen -dev packages, but I apparently still don't have enough.

Program received signal SIGSEGV, Segmentation fault.
gdk_window_new (parent=0x9117e0, attributes=0x7fffffffb050, attributes_mask=44) at /build/buildd/gtk+3.0-3.2.0/./gdk/gdkwindow.c:1359
1359    /build/buildd/gtk+3.0-3.2.0/./gdk/gdkwindow.c: No such file or directory.
        in /build/buildd/gtk+3.0-3.2.0/./gdk/gdkwindow.c


#0  gdk_window_new (parent=0x9117e0, attributes=0x7fffffffb050, attributes_mask=44) at /build/buildd/gtk+3.0-3.2.0/./gdk/gdkwindow.c:1359
#1  0x00007fffe23eebb5 in ?? () from /usr/lib/libido3-0.1.so.0
#2  0x00007ffff69570a4 in g_closure_invoke () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#3  0x00007ffff696881a in ?? () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#4  0x00007ffff69726b1 in g_signal_emit_valist () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#5  0x00007ffff6972852 in g_signal_emit () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#6  0x00007ffff72c3190 in gtk_widget_realize (widget=0x8dc3c0) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkwidget.c:4401
#7  0x00007ffff72c3e28 in gtk_widget_map (widget=0x8dc3c0) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkwidget.c:4171
#8  0x00007ffff70c2d34 in gtk_box_forall (container=<optimized out>, include_internals=<optimized out>, callback=0x7ffff7100b10 <gtk_container_map_child>,
    callback_data=0x0) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkbox.c:1856
#9  0x00007ffff710447f in gtk_container_map (widget=0x8dc480) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkcontainer.c:3248
#10 0x00007ffff6956ffa in g_closure_invoke () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#11 0x00007ffff696881a in ?? () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#12 0x00007ffff69726b1 in g_signal_emit_valist () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#13 0x00007ffff6972852 in g_signal_emit () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#14 0x00007ffff72c3d1a in gtk_widget_map (widget=0x8dc480) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkwidget.c:4173
#15 0x00007ffff710447f in gtk_container_map (widget=0x993050) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkcontainer.c:3248
#16 0x00007ffff719ac72 in gtk_menu_item_map (widget=<optimized out>) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmenuitem.c:1651
#17 0x00007ffff6956ffa in g_closure_invoke () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#18 0x00007ffff696881a in ?? () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#19 0x00007ffff69726b1 in g_signal_emit_valist () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#20 0x00007ffff6972852 in g_signal_emit () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#21 0x00007ffff72c3d1a in gtk_widget_map (widget=0x993050) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkwidget.c:4173
#22 0x00007ffff719e17d in gtk_menu_shell_forall (container=<optimized out>, include_internals=<optimized out>,
    callback=0x7ffff7100b10 <gtk_container_map_child>, callback_data=0x0) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmenushell.c:1264
#23 0x00007ffff710447f in gtk_container_map (widget=0x86a6e0) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkcontainer.c:3248
#24 0x00007ffff6956ffa in g_closure_invoke () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#25 0x00007ffff696881a in ?? () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#26 0x00007ffff69726b1 in g_signal_emit_valist () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#27 0x00007ffff6972852 in g_signal_emit () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#28 0x00007ffff72c3d1a in gtk_widget_map (widget=0x86a6e0) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkwidget.c:4173
#29 0x00007ffff72d5960 in gtk_window_map (widget=0x651a10) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkwindow.c:4766
#30 0x00007ffff69570a4 in g_closure_invoke () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#31 0x00007ffff696881a in ?? () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#32 0x00007ffff69726b1 in g_signal_emit_valist () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#33 0x00007ffff6972852 in g_signal_emit () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#34 0x00007ffff72c3d1a in gtk_widget_map (widget=0x651a10) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkwidget.c:4173
#35 0x00007ffff72cdde4 in gtk_window_show (widget=0x651a10) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkwindow.c:4705
#36 0x00007ffff69570a4 in g_closure_invoke () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#37 0x00007ffff696881a in ?? () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#38 0x00007ffff69726b1 in g_signal_emit_valist () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#39 0x00007ffff6972852 in g_signal_emit () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#40 0x00007ffff72c2610 in gtk_widget_show (widget=0x651a10) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkwidget.c:3905
#41 gtk_widget_show (widget=0x651a10) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkwidget.c:3882
#42 0x00007ffff7195074 in gtk_menu_popup_for_device (menu=0x86a6e0, device=<optimized out>, parent_menu_shell=0x65c7e0, parent_menu_item=0x7bd510,
    func=0x7ffff7199050 <gtk_menu_item_position_menu>, data=0x7bd510, destroy=0, button=1, activate_time=0) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmenu.c:1721
#43 0x00007ffff719557b in gtk_menu_popup (menu=0x86a6e0, parent_menu_shell=0x65c7e0, parent_menu_item=0x7bd510,
    func=0x7ffff7199050 <gtk_menu_item_position_menu>, data=0x7bd510, button=1, activate_time=0) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmenu.c:1785
#44 0x00007ffff7198f29 in gtk_menu_item_real_popup_submenu (widget=0x7bd510, remember_exact_time=<optimized out>)
    at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmenuitem.c:2001
#45 0x00007ffff719c41e in _gtk_menu_item_popup_submenu (widget=0x7bd510, with_delay=<optimized out>) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmenuitem.c:2100
#46 0x00007ffff719c4d2 in gtk_real_menu_item_select (menu_item=0x7bd510) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmenuitem.c:1809
#47 0x00007ffff69570a4 in g_closure_invoke () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#48 0x00007ffff696881a in ?? () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#49 0x00007ffff69726b1 in g_signal_emit_valist () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#50 0x00007ffff6972852 in g_signal_emit () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#51 0x00007ffff719c0aa in gtk_menu_item_select (menu_item=0x7bd510) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmenuitem.c:1382
#52 0x00007ffff71a06b8 in gtk_menu_shell_real_select_item (menu_shell=0x65c7e0, menu_item=0x7bd510) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmenushell.c:1407
#53 0x00007ffff71a09ad in gtk_menu_shell_button_press (event=0x7fffe8047000, widget=0x65c7e0) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmenushell.c:789
#54 gtk_menu_shell_button_press (widget=0x65c7e0, event=0x7fffe8047000) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmenushell.c:734
#55 0x00007ffff7189e58 in _gtk_marshal_BOOLEAN__BOXED (closure=0x62c3d0, return_value=0x7fffffffd490, n_param_values=<optimized out>,
---Type <return> to continue, or q <return> to quit---
    param_values=0x7fffe803a460, invocation_hint=<optimized out>, marshal_data=<optimized out>) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmarshalers.c:85
#56 0x00007ffff69570a4 in g_closure_invoke () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#57 0x00007ffff6968e5f in ?? () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#58 0x00007ffff6972483 in g_signal_emit_valist () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#59 0x00007ffff6972852 in g_signal_emit () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#60 0x00007ffff72b7f39 in gtk_widget_event_internal (widget=0x65c7e0, event=0x7fffe8047000) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkwidget.c:6132
#61 0x00007ffff718963a in gtk_propagate_event (widget=0x65c7e0, event=0x7fffe8047000) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmain.c:2629
#62 0x00007ffff7189a3b in gtk_main_do_event (event=0x7fffe8047000) at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmain.c:1894
#63 0x00007ffff6e02102 in gdk_event_source_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>)
    at /build/buildd/gtk+3.0-3.2.0/./gdk/x11/gdkeventsource.c:360
#64 0x00007ffff6696a5d in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#65 0x00007ffff6697258 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#66 0x00007ffff6697792 in g_main_loop_run () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#67 0x00007ffff7188e1d in gtk_main () at /build/buildd/gtk+3.0-3.2.0/./gtk/gtkmain.c:1367
#68 0x00007ffff789b0c3 in ?? () from /usr/lib/libpanel-applet-4.so.0
#69 0x0000000000403155 in ?? ()
#70 0x00007ffff60b730d in __libc_start_main () from /lib/x86_64-linux-gnu/libc.so.6
#71 0x00000000004031a1 in ?? ()
#72 0x00007fffffffda68 in ?? ()
#73 0x000000000000001c in ?? ()
#74 0x0000000000000001 in ?? ()
#75 0x00007fffffffdde2 in ?? ()
#76 0x0000000000000000 in ?? ()