Patch does work.

Video: https://youtu.be/61PxJJv7pxo (BUG)

[Test Case #1]
Dell Inspiron 15-3567, Intel Core i5 7th Gen, for external monitor/TV used 
Samsung 1920x TV.

1) Start up laptop and login with Cinnamon
2) Plug in TV (external monitor), HDMI cable
3) Open display cinnamon settings module
4) Arrange the laptop monitor ON THE RIGHT OF THE EXTERNAL MONITOR
5) Set the TV as the primary display.
6) Close the laptop lid. This will turn off the laptop monitor.
7) Open some windows, play around with the TV monitor for a bit.
8) Reopen the laptop monitor.
9) Graphics will lag and cause black borders and screen tearing. In this case 
it was on the laptop monitor but some other times when I tried it happened on 
the TV.

[Test Case #1 Notes]
This is WITHOUT THE PATCH.
I noticed that you dont even have to turn off and re-turn on the laptop monitor 
for the screen tearing/black borders to occur.
Other times the laptop monitor would just turn off and you wouldn't be able to 
turn it back on unless you forced the Display settings module to make the 
monitor active.
Apport or Ubuntu-bug didn't popup a window showing of a crash.
Black borders at 2:15 in video.

[TEST CASE #2]
10) Software and Updates -> Development -> Enable -proposed repos
11) Refresh cache
12) Open terminal
13) sudo apt update, sudo apt upgrade
14) Reboot
15) Login and try above bug replication again
16) No errors/bugs.

** Description changed:

  This is an upstream issue-read the following bug reports.
  
  https://github.com/linuxmint/cinnamon/issues/9401
  https://github.com/linuxmint/cinnamon/issues/9468
  
  An SRU patch is now available.
  
  This is a REGRESSION of a commit in Cinnamon 4.0.0
  
(https://github.com/linuxmint/muffin/commit/1d38120c9ded2578a14cb2e8bf46bb82b3acdc75)
  
  Debian Bug Patch: https://bugs.debian.org/cgi-
  bin/bugreport.cgi?bug=965382
  
  SRU TEMPLATE:
  
  [Impact]
  
   * In fallback mode Cinnamon/Muffin segfaults and makes the desktop
  unusable.
  
   * This would occur by having two monitors-a laptop and an external with the 
laptop off and the external on-supposedly with NVIDIA as a driver.
  [Test Case]
  
   * Upstream commits example to replicate bug:
   "Example to reproduce: Laptop monitor + external monitor.  Arrange laptop
   monitor to the right of the external one.  Using nvidia-450 (confirmed
   for me, at least), turn laptop monitor off.  Ignoring horrible lag from
   driver, turn the laptop monitor back on (arranging it still to the right
   of the external monitor)."
  
  [Regression Potential]
  
  * Regressions manifesting is rather barely likely. Latest versions of Muffin 
patches this and so is upstream unstable have this patched. Regressions 
shouldn't be a threat unless:
  1) Dependency problem
  2) New versions of C causing incompatability problems or versions of 
Gobject-Introspection
  
   * In the event of a regression, which is unlikely as of Aug 13 (UTC),
  as Muffin's update would've been released a while ago (24 days), 2 days
  ago the version was bumped. A new upstream version would likely patch
  this problem aswell.
  
  * After 24 days it seems there is nothing to worry about regarding
  regressions but of course, always something to worry about.
  
  * It's been about enough time, nearly a month to spot any regressions
  from this fix.
  
   * This is actually a SOLVE of a regression
  
   * This regression fix goes back to Cinnamon 4.0.0-or when Eoan Ermine
  was released by getting refresh rates form xrandr
  
   * In upstream Mint this was an SRU/release as seen here:
  https://github.com/linuxmint/muffin/releases/tag/master.mint20 (The
  latest is the patch for this bug)
  
  [Other Info]
  
   * I couldn't pbuilder-dist for some reason. Missing dependencies or
  stuff didn't exist-in sbuild it did though and my debuild -S did.
  
   * I will work on patching this in Groovy through debian unstable and
  will  attach the bug report for groovy as part of this
  situation/regression.
  
+ 
+ Video: https://youtu.be/61PxJJv7pxo (BUG)
+ 
+ [Test Case #1]
+ Dell Inspiron 15-3567, Intel Core i5 7th Gen, for external monitor/TV used 
Samsung 1920x TV.
+ 
+ 1) Start up laptop and login with Cinnamon
+ 2) Plug in TV (external monitor), HDMI cable
+ 3) Open display cinnamon settings module
+ 4) Arrange the laptop monitor ON THE RIGHT OF THE EXTERNAL MONITOR
+ 5) Set the TV as the primary display.
+ 6) Close the laptop lid. This will turn off the laptop monitor.
+ 7) Open some windows, play around with the TV monitor for a bit.
+ 8) Reopen the laptop monitor.
+ 9) Graphics will lag and cause black borders and screen tearing. In this case 
it was on the laptop monitor but some other times when I tried it happened on 
the TV.
+ 
+ [Test Case #1 Notes]
+ This is WITHOUT THE PATCH.
+ I noticed that you dont even have to turn off and re-turn on the laptop 
monitor for the screen tearing/black borders to occur.
+ Other times the laptop monitor would just turn off and you wouldn't be able 
to turn it back on unless you forced the Display settings module to make the 
monitor active.
+ Apport or Ubuntu-bug didn't popup a window showing of a crash.
+ Black borders at 2:15 in video.
+ 
+ [TEST CASE #2]
+ 10) Software and Updates -> Development -> Enable -proposed repos
+ 11) Refresh cache
+ 12) Open terminal
+ 13) sudo apt update, sudo apt upgrade
+ 14) Reboot
+ 15) Login and try above bug replication again
+ 16) No errors/bugs.
+ 
+ VERIFICATION DONE
+ Patch does work-without the patch with the dual-monitor you would get instant 
black bordering. This patch removes it.
+ 
+ I emailed Michael Webster from upstream about the patch. This was his
+ response:
+ 
+ "
+ - Immediately below the code insertion is a check that info != NULL, so the 
code prior to that commit allowed that info could be NULL (this code was 
existing from our original commit when we forked mutter ~9 years ago).
+ - The crash was specifically due to attempting to dereference a NULL info.
+ "
+ He hasn't tested previous versions but I have confirmed 4.8, the only one 
Ubuntu is maintaining for now and the regression was introduced in 4.0 (Eoan). 
+ 
  ProblemType: Bug
  DistroRelease: Ubuntu 20.04
  Package: muffin 4.4.3-1
  ProcVersionSignature: Ubuntu 5.4.0-40.44-generic 5.4.44
  Uname: Linux 5.4.0-40-generic x86_64
  ApportVersion: 2.20.11-0ubuntu27.3
  Architecture: amd64
  CasperMD5CheckResult: skip
  CurrentDesktop: ubuntu:GNOME
  Date: Sun Jul 19 21:49:09 2020
  InstallationDate: Installed on 2020-04-23 (87 days ago)
  InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
  ProcEnviron:
   TERM=xterm-256color
   PATH=(custom, no user)
   XDG_RUNTIME_DIR=<set>
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  SourcePackage: muffin
  UpgradeStatus: No upgrade log present (probably fresh install)

** Tags removed: verification-needed verification-needed-focal
** Tags added: verification-done verification-done-focal

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https://bugs.launchpad.net/bugs/1888169

Title:
  Cinnamon segfault at 28 ip 00007fb707b32ae2 sp 00007ffd444e3780 error
  6 in libmuffin.so.0.0.0[7fb707afe000+74000]

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