Upgrade window freezes when I press Ctrl+Shift+I

I was upgrading my Ubuntu GNOME 15.10 system to 16.04 with update-manager.

Note: I've the org.gtk.Settings.Debug enable-inspector-keybinding gsetting set to true to enable the GTK+ Inspector.

I hit Ctrl+Shift+I (the key to open the GTK+ inspector) while the upgrade window was focused, in the middle of upgrading packages. A few minutes later the upgrade process stopped making progress (CPU idle, no disk I/O). The window is not reacting to mouse clicks or key events.

pstree shows

   xenial,16938,root /tmp/ubuntu-release-upgrader-_yjo7xr4/xenial --mode=server --frontend=DistUpgradeViewGtk3
     `-(dpkg,6263)

and strace -p 16938 shows

   write(14, "pmstatus:libde265-0:16.8172:Installing libde265-0 (amd64)\n", 58

and lsof -p 16938 shows

  xenial 16938 root 13r FIFO 0,10 0t0 5172320 pipe
  xenial 16938 root 14w FIFO 0,10 0t0 5172320 pipe

while the last few terminal messages shown in the upgrade window were

  Unpacking gir1.2-atspi-2.0 (2.18.3-4ubuntu1) over (2.16.0-1ubuntu1) ...
  Processing triggers for ureadahead (0.100.0-19) ...
  Processing triggers for man-db (2.7.4-1) ...

/var/log/dist-upgrade/apt-term.log ends has those lines, and more (507 lines or 53773 bytes more), ending in

  Preparing to unpack .../libde265-0_1.0.2-2_amd64.deb ...

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:15.10.14.4
ProcVersionSignature: Ubuntu 4.2.0-35.40-generic 4.2.8-ckt5
Uname: Linux 4.2.0-35-generic x86_64
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: GNOME
Date: Fri Apr 22 14:44:18 2016
EcryptfsInUse: Yes
InstallationDate: Installed on 2012-07-25 (1366 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
Symptom: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2016-04-22 (0 days ago)
VarLogDistupgradeAptclonesystemstate.tar.gz: Error: command ['pkexec', 'cat', '/var/log/dist-upgrade/apt-clone_system_state.tar.gz'] failed with exit code 127: pkexec must be setuid root
VarLogDistupgradeTermlog: