Please see screenshots of "PreferencesDialog.ui" opened in Qt Designer 5.9.7 and Qt Designer 5.12.
- Horizontal layout with the "Language" label is broken: the label is not vertically centered, also the layout is taller for some reason
- The QTabWidget control borders are not correctly drawn for some reason (when compiled in an app, it's fine though)
PS. For some reason
QTBUG-71958 is closed but never fixed.