bionic -> cosmic dist-upgrade ends up with graphical Shell artefacts

[Didier Roche-Tolomelli]

Preliminary note: I can't reproduce it anymore since 11th October 2018. Was hitting it in my VM 100% beforehand though. The VM was set at 1Gb (default Boxes value at the time), so maybe hitting OOM killing too hard. I can't dist-upgrade anymore with 1Gb of memory (VM hangs) anymore, so could be that or package installation order.

This is why the proposed patch is just posted right now, but not applied.
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On bionic, run do-release-upgrade
At 2/3 of the installation, you will see icons disappearing from the dock and the system Shell menu, blue background, menu entries missing… (see attached picture).
After reboot, on cosmic, you end up with Yaru theme. The issue is only during/after the dist-upgrade upon reboot.
-> Restarting the Shell loads the default Shell theme (not Yaru) as we address now an ubuntu.css file which doesn't exists. All icons, theme, and menu entries are restored.

I have attached a potential patch which triggerts the Shell restart in ubuntu-release-upgrader (before obsolete package cleanup) as a Quirk (Xorg-only), which is tested.
However, as of today I can't reproduce this issue and unsure if this one was due to low memory or package ordering, I wonder if it worths pushing this patch.
It seems conscensus on #ubuntu-desktop is to wait and see.

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: ubuntu-release-upgrader-core 1:18.10.10
ProcVersionSignature: Ubuntu 4.18.0-8.9-generic 4.18.7
Uname: Linux 4.18.0-8-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.10-0ubuntu11
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Thu Oct 11 11:56:25 2018
InstallationDate: Installed on 2018-05-24 (139 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
Symptom: ubuntu-release-upgrader
UpgradeStatus: No upgrade log present (probably fresh install)