Here's the crash with GDK_SYNCHRONIZE=1. It appears to be graphical. And I have a theory this maybe only happens on machines that default to hi-DPI (scale 200%) in the live session...

#6  0x00007fffef656c8d in _XError () from /usr/lib/x86_64-linux-gnu/libX11.so.6
#7  0x00007fffef653bb7 in ?? () from /usr/lib/x86_64-linux-gnu/libX11.so.6
#8  0x00007fffef653c75 in ?? () from /usr/lib/x86_64-linux-gnu/libX11.so.6
#9  0x00007fffef654b88 in _XReply () from /usr/lib/x86_64-linux-gnu/libX11.so.6
#10 0x00007fffef65057d in XSync () from /usr/lib/x86_64-linux-gnu/libX11.so.6
#11 0x00007fffef65061b in ?? () from /usr/lib/x86_64-linux-gnu/libX11.so.6
#12 0x00007fffef65760f in ?? () from /usr/lib/x86_64-linux-gnu/libX11.so.6
#13 0x00007fffef40d7b7 in XShmAttach ()
   from /usr/lib/x86_64-linux-gnu/libXext.so.6
#14 0x00007fffef18b82b in ?? () from /usr/lib/x86_64-linux-gnu/libcairo.so.2
#15 0x00007fffef18c05e in ?? () from /usr/lib/x86_64-linux-gnu/libcairo.so.2
#16 0x00007fffef189c81 in ?? () from /usr/lib/x86_64-linux-gnu/libcairo.so.2
#17 0x00007fffef1868e8 in ?? () from /usr/lib/x86_64-linux-gnu/libcairo.so.2
#18 0x00007fffef16a585 in ?? () from /usr/lib/x86_64-linux-gnu/libcairo.so.2
---Type <return> to continue, or q <return> to quit---
#19 0x00007fffef16a78c in ?? () from /usr/lib/x86_64-linux-gnu/libcairo.so.2
#20 0x00007fffef16ae28 in ?? () from /usr/lib/x86_64-linux-gnu/libcairo.so.2
#21 0x00007fffef10e021 in ?? () from /usr/lib/x86_64-linux-gnu/libcairo.so.2
#22 0x00007fffef157efc in ?? () from /usr/lib/x86_64-linux-gnu/libcairo.so.2
#23 0x00007fffef11657e in ?? () from /usr/lib/x86_64-linux-gnu/libcairo.so.2
#24 0x00007fffef10fb39 in ?? () from /usr/lib/x86_64-linux-gnu/libcairo.so.2
#25 0x00007fffef108a55 in cairo_fill ()
   from /usr/lib/x86_64-linux-gnu/libcairo.so.2
#26 0x00007fffc2e66240 in ?? ()
   from /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37
#27 0x00007fffc1fd1e61 in ?? ()
   from /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37
#28 0x00007fffc2051a3b in ?? ()
   from /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37
#29 0x00007fffc1fdbbd1 in ?? ()
   from /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37
#30 0x00007fffc1fe1897 in ?? ()
   from /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37
#31 0x00007fffc1f4879f in ?? ()
   from /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37
#32 0x00007fffc1ccabde in ?? ()
   from /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37
#33 0x00007fffc1daeed2 in ?? ()
---Type <return> to continue, or q <return> to quit---
   from /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37
#34 0x00007fffc1cc67ab in ?? ()
   from /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37
#35 0x00007fffc1cc7095 in ?? ()
   from /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37
#36 0x00007fffc1005903 in WTF::RunLoop::performWork() ()
   from /usr/lib/x86_64-linux-gnu/libjavascriptcoregtk-4.0.so.18
#37 0x00007fffc102e959 in ?? ()
   from /usr/lib/x86_64-linux-gnu/libjavascriptcoregtk-4.0.so.18
#38 0x00007ffff4b9e0f5 in g_main_context_dispatch ()
   from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#39 0x00007ffff4b9e4c0 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#40 0x00007ffff4b9e7d2 in g_main_loop_run ()
   from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#41 0x00007fffec971385 in gtk_main ()
   from /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#42 0x00007ffff494fdae in ffi_call_unix64 ()
   from /usr/lib/x86_64-linux-gnu/libffi.so.6
#43 0x00007ffff494f71f in ffi_call ()
   from /usr/lib/x86_64-linux-gnu/libffi.so.6
#44 0x00007ffff531bdda in ?? ()
   from /usr/lib/python3/dist-packages/gi/_gi.cpython-36m-x86_64-linux-gnu.so
#45 0x00007ffff531d9b8 in ?? ()
---Type <return> to continue, or q <return> to quit---
   from /usr/lib/python3/dist-packages/gi/_gi.cpython-36m-x86_64-linux-gnu.so
#46 0x000000000045a0e3 in _PyObject_FastCallDict ()
#47 0x000000000054fd37 in ?? ()
#48 0x00000000005546cf in _PyEval_EvalFrameDefault ()