The title says it all. If you choose a custom font size for the tree view (because fonts in ubuntu are too big, and this program's interface is already quite cluttered), the next time you start Digikam the font goes back to default. Just entering and clicking "Accept" on the settings menu is enough to bring back the custom font settings, but it needs to be done on each startup. I experienced it on Ubuntu 16.04.3 LTS and Digikam 5.8.0 (but was also present in Digikam 5.7.0), using the appimage version.
Git commit daf1bc88f4def4257f7ad8211bc30ea08fd3ccb7 by Maik Qualmann. Committed on 30/01/2018 at 19:03. Pushed by mqualmann into branch 'master'. fix not applied custom font to the AbstractAlbumTreeView FIXED-IN: 5.9.0 M +2 -1 NEWS M +1 -0 libs/album/albumtreeview.cpp https://commits.kde.org/digikam/daf1bc88f4def4257f7ad8211bc30ea08fd3ccb7