Bug#945562: libqt5core5a: Update to 5.12 breaks KDE login

[David Engel <david@istwok.net>]

Package: libqt5core5a
Version: 5.12.5+dfsg-2
Severity: important

Dear Maintainer,

I had not rebooted my Deiban/testing system since mid-October.  When I
did, my KDE session was unustable.  After entering my password into
sddm, the screen began flashing wildly with my restored session
windows visible between the flashes.  The end result was a mish-mash
of the KDE login animation and my restored windows and a system that
was unresponsive to keyboard input and mouse clicks.

After much trial and error and several system restores from backups, I
narrowed the problem down to upgrading from qt version 5.11.3+dsfg1-3
to version 5.12.5+dsfg-2.  I suppose there is a slight chance that the
latest versions of libkf5xmlgui and plasma-integration are also to
blame since the qt upgrade forced them to be upgraded too.

I'm currently forced to use XFCE since KDE is unusable.
Interestingly, KDE programs like konsole and dolphin show no ill
effects when run under XFCE.  The mythtv installation I build myself
and uses Qt and OpenGL also works fine.

The problem does not appear to have anything to do with my personal
KDE configuration.  I created a dummy user with no existing
configuration and the problem still occurred.

Finally, the problem does not occur on another system that I maintain
which uses the nvidia-legacy-390xx graphics driver.  Perhaps the
problem is related to something in the intel graphics driver on this
old, Ivy Bridge system.

Please let me know if there is any other information you want me to
provide like logs or things you want me to try.

David

-- System Information:
Debian Release: bullseye/sid
  APT prefers testing
  APT policy: (900, 'testing'), (100, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 5.3.0-2-amd64 (SMP w/8 CPU cores)
Kernel taint flags: TAINT_OOT_MODULE, TAINT_UNSIGNED_MODULE
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE= (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages libqt5core5a depends on:
ii  libc6                  2.29-3
ii  libdouble-conversion3  3.1.5-3
ii  libgcc1                1:9.2.1-19
ii  libglib2.0-0           2.62.2-3
ii  libicu63               63.2-2
ii  libpcre2-16-0          10.32-5+b1
ii  libstdc++6             9.2.1-19
ii  zlib1g                 1:1.2.11.dfsg-1+b1

Versions of packages libqt5core5a recommends:
pn  qttranslations5-l10n  <none>

Versions of packages libqt5core5a suggests:
ii  libthai0  0.1.28-3

-- no debconf information