bug#54033: Calibre's ebook-viewer only shows white-on-white or dark-on-dark.

[Jacob MacDonald]

My description of the bug was hasty. The viewer is still broken in the
same way, but I realize now that the text is not invisible or
unselectable. Rather, selection still works but only displays
selection markers at the upper left corner of the screen. According to
a bug report about Calibre, similar things can happen if the fonts are
not properly installed
(https://www.mobileread.com/forums/showthread.php?t=315146). However,
I checked a working version and broken version of my Guix profile
against one another and the fonts appear to be installed correctly in
both trees.

While doing that, I also noticed that __pycache__ directories only
exist in the broken tree. Is that a signal of something going wrong or
just a change in Guix's Python?