Don't know if I did this right...
~ $ gdb gtk-window-decorator
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Reading symbols from gtk-window-decorator...(no debugging symbols found)...done.
(gdb) run
Starting program: /usr/bin/gtk-window-decorator
warning: Could not load shared library symbols for linux-vdso.so.1.
Do you need "set solib-search-path" or "set sysroot"?
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
[New Thread 0x7fffe9290700 (LWP 16811)]
[New Thread 0x7fffe8882700 (LWP 16812)]
(gtk-window-decorator:16807): Pango-CRITICAL **: pango_font_description_from_string: assertion 'str != NULL' failed
(gtk-window-decorator:16807): Pango-CRITICAL **: pango_font_description_get_size: assertion 'desc != NULL' failed
(gtk-window-decorator:16807): Pango-CRITICAL **: pango_font_description_set_size: assertion 'desc != NULL' failed
(gtk-window-decorator:16807): Gtk-CRITICAL **: gtk_style_context_get_property: assertion 'priv->widget != NULL || priv->widget_path != NULL' failed
(gtk-window-decorator:16807): GLib-GObject-WARNING **: gtype.c:4221: type id '0' is invalid
(gtk-window-decorator:16807): GLib-GObject-WARNING **: can't peek value table for type '<invalid>' which is not currently referenced
Program received signal SIGSEGV, Segmentation fault.
0x00007ffff6e09aa3 in gtk_style_context_get_valist () from /usr/lib/libgtk-3.so.0
(gdb) bt
#0 0x00007ffff6e09aa3 in gtk_style_context_get_valist () from /usr/lib/libgtk-3.so.0
#1 0x00007ffff6e09d63 in gtk_style_context_get () from /usr/lib/libgtk-3.so.0
#2 0x000000000041b89b in frame_update_titlebar_font ()
#3 0x0000000000410ab3 in decor_frame_refresh ()
#4 0x0000000000410c32 in gwd_get_decor_frame ()
#5 0x000000000041ce88 in update_default_decorations ()
#6 0x0000000000419c10 in decorations_changed ()
#7 0x000000000041e1dc in ?? ()
#8 0x00007ffff5b0fedd in g_list_foreach () from /usr/lib/libglib-2.0.so.0
#9 0x000000000041fb6a in ?? ()
#10 0x000000000040d967 in main ()
(gdb) quit
A debugging session is active.
Inferior 1 [process 16807] will be killed.