linker error on compilation, e-mailed maintainer
Linking CXX executable babe
CMakeFiles/babe.dir/mainwindow.cpp.o: In function `MainWindow::setUpViews()':
mainwindow.cpp:(.text+0x135b): undefined reference to `OnlineView::OnlineView(QWidget*)'

In T5884#89880, @jriddell wrote:

linker error on compilation, e-mailed maintainer
Linking CXX executable babe
CMakeFiles/babe.dir/mainwindow.cpp.o: In function `MainWindow::setUpViews()':
mainwindow.cpp:(.text+0x135b): undefined reference to `OnlineView::OnlineView(QWidget*)'

Confirmed. We were getting successful builds on KCI until today's commit:

https://cgit.kde.org/babe.git/commit/?id=77a5d55589694adbc1a9006baca6e99b95650d43

I tried building with qmake instead, as it seems the babe maintainer uses this and does not much focus on successful builds with cmake. This works for Kubuntu with GCC on Artful and Bionic, but fails for Xenial. So reverted changes.

Ultimately it's better if the babe developer fixes his cmake build I think ;)