meld assert failure: python3: ../../src/xcb_io.c:259: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.

[Nicolas_Raoul]

I was using Meld and placing several pairs of synchronization points (see https://stackoverflow.com/q/21571173/226958 about what these are) when Meld froze. UI was not responding at all, and the following appeared in the terminal:

[xcb] Unknown sequence number while processing queue
[xcb] Most likely this is a multi-threaded client and XInitThreads has not been called
[xcb] Aborting, sorry about that.
python3: ../../src/xcb_io.c:259: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.

ProblemType: Crash
DistroRelease: Ubuntu 18.04
Package: meld 3.18.0-6
ProcVersionSignature: Ubuntu 4.15.0-22.24-generic 4.15.17
Uname: Linux 4.15.0-22-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
AssertionMessage: python3: ../../src/xcb_io.c:259: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.
CurrentDesktop: ubuntu:GNOME
Date: Tue Jun 12 14:40:19 2018
ExecutablePath: /usr/bin/meld
InstallationDate: Installed on 2018-04-29 (43 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
InterpreterPath: /usr/bin/python3.6
PackageArchitecture: all
ProcCmdline: /usr/bin/python3 /usr/bin/meld /tmp/mine.log /tmp/you.txt
Python3Details: /usr/bin/python3.6, Python 3.6.5, python3-minimal, 3.6.5-3
PythonDetails: /usr/bin/python2.7, Python 2.7.15rc1, python-minimal, 2.7.15~rc1-1
Signal: 6
SourcePackage: meld
StacktraceTop:
 __assert_fail_base (fmt=0x7f867c4947d8 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=assertion@entry=0x7f86767a2660 "!xcb_xlib_threads_sequence_lost", file=file@entry=0x7f86767a24cb "../../src/xcb_io.c", line=line@entry=259, function=function@entry=0x7f86767a2908 "poll_for_event") at assert.c:92
 __GI___assert_fail (assertion=0x7f86767a2660 "!xcb_xlib_threads_sequence_lost", file=0x7f86767a24cb "../../src/xcb_io.c", line=259, function=0x7f86767a2908 "poll_for_event") at assert.c:101
 () at /usr/lib/x86_64-linux-gnu/libX11.so.6
 () at /usr/lib/x86_64-linux-gnu/libX11.so.6
 _XEventsQueued () at /usr/lib/x86_64-linux-gnu/libX11.so.6
Title: meld assert failure: python3: ../../src/xcb_io.c:259: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip docker lpadmin plugdev sambashare sudo wireshark

[Apport retracing service (apport)]

StacktraceTop:
 __assert_fail_base (fmt=0x7f867c4947d8 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=assertion@entry=0x7f86767a2660 "!xcb_xlib_threads_sequence_lost", file=file@entry=0x7f86767a24cb "../../src/xcb_io.c", line=line@entry=259, function=function@entry=0x7f86767a2908 <__PRETTY_FUNCTION__.14350> "poll_for_event") at assert.c:92
 __GI___assert_fail (assertion=assertion@entry=0x7f86767a2660 "!xcb_xlib_threads_sequence_lost", file=file@entry=0x7f86767a24cb "../../src/xcb_io.c", line=line@entry=259, function=function@entry=0x7f86767a2908 <__PRETTY_FUNCTION__.14350> "poll_for_event") at assert.c:101
 poll_for_event (dpy=dpy@entry=0xdc5000) at ../../src/xcb_io.c:256
 poll_for_response (dpy=dpy@entry=0xdc5000) at ../../src/xcb_io.c:274
 _XEventsQueued (dpy=dpy@entry=0xdc5000, mode=mode@entry=1) at ../../src/xcb_io.c:349