Just for the sake of completion I will put a more detailed explanation of the issue I uncovered on my system and the choice of fixes to rectify the issue.
I run Fluxbox, a very minimalist desktop, which as it turns out does not adequately set / show the DPI for my display. Other DEs available in a full installation of Slackware (Xfce and Plasma / KDE) do this automatically.
It looks like QT/Calibre automatically sets size of display, fonts etc by reading values relating to display, values which were not available on my Fluxbox setup. A bizarrely large value is then looked for which causes the error that I saw and the eventual segmentation fault of Calibre.
Solutions are all local and their are 2 choices:
- Fix the display values on my system: the correct DPI can be set with xrandr easily enough, or entered into ~/.Xresource or any of several other choices.
- Block the ability of QT/Calibre to auto-size at all by using QT_AUTO_SCREEN_SCALE_FACTOR=0 and perhaps allowing a fixed resize by using QT_SCALE_FACTOR=xxx.
I am happy now, I have learnt something new and I have the latest version of Calibre open even as I am typing
.