The bundled libfontconfig seems to cause problems when the target system uses
a newer font config format. See discussion here:
https://mail.kde.org/pipermail/kdevelop/2018-October/019726.html
It seems to be a common problem (as seen in bug reports of other AppImage
projects).
It's also blacklisted in linuxdeployqt:
https://github.com/AppImage/AppImages/blob/master/excludelist
For me locally, after the change I also have the exact same font appearance
as with native applications. Before the patch the fonts did use my
anti-aliasing settings iiuc. Before the patch, running the AppImage resulted
in lots of warnings:
Fontconfig warning: line 5: unknown element "its:rules" Fontconfig warning: line 6: unknown element "its:translateRule" Fontconfig warning: line 9: unknown element "description" Fontconfig warning: "/etc/fonts/conf.d/10-hinting-slight.conf", line 4: unknown element "its:rules" Fontconfig warning: "/etc/fonts/conf.d/10-hinting-slight.conf", line 5: unknown element "its:translateRule" Fontconfig warning: "/etc/fonts/conf.d/10-hinting-slight.conf", line 8: unknown element "description" ...
... these warnings are now gone.
Thanks for the analysis by Friedrich.
Actually easy to test using the new AppImage format. Just do:
./KDevelop-5.3.0-x86_64.AppImage --appimage-extract cd squashfs-root ./AppRun # to check appearance before rm rm usr/lib/libfontconfig.so.1 ./AppRun # to check appearance after rm'ing the lib