SUMMARY
I get a segfault on long canvas sizes. Interestingly the fault is not only depending on the canvas size but also on its content. Here a working example and the one with the bug:

// Bug (seg. fault)
QMessageBox::question(qApp->activeWindow(),
                      QObject::tr("Saving"),
                      QObject::tr("Remember that when you move or rename image files on your disk or the storage device become unavailable"));

// No Bug (just '\n' added)
QMessageBox::question(qApp->activeWindow(),
                      QObject::tr("Saving"),
                      QObject::tr("Remember that when you move or rename image files\n on your disk or the storage device become unavailable"));

See https://github.com/cgilles/digikam-photolayouteditor/commit/1ded36d366884733b89753fb06e50e91396ce48b?branch=1ded36d366884733b89753fb06e50e91396ce48b&diff=unified for the origin of this bug

See also the attached backtrace

STEPS TO REPRODUCE
1. Run a program with the code above

OBSERVED RESULT
seg. fault

EXPECTED RESULT
No segfault. Support of longer lines.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 20.04
(available in About System)
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

ADDITIONAL INFORMATION