Comment 0 for bug 1926737

Lack of permissions to access the display causes X11 to dump core.

You get

Qt WARNING: could not connect to display
Qt FATAL: This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: wayland-org.kde.kwin.qpa, eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, xcb.

Aborted (core dumped)

but this should not be a condition to abort with a core dump, rather a mere error on which the application should exit (as I believe it used to do until not long ago)

To trigger the issue it is enough to be on X11, su to another user (not having permission to use the display) try to launch a QT application from that user.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: libqt5core5a 5.12.8+dfsg-0ubuntu1
ProcVersionSignature: Ubuntu 5.8.0-50.56~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-50-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: KDE
Date: Fri Apr 30 11:31:57 2021
EcryptfsInUse: Yes
InstallationDate: Installed on 2020-02-16 (439 days ago)
InstallationMedia: Kubuntu 19.10 "Eoan Ermine" - Release amd64 (20191017)
SourcePackage: qtbase-opensource-src
UpgradeStatus: Upgraded to focal on 2020-05-23 (342 days ago)