KWin's TabBox (Alt+Tab) implementation uses a QAbstractItemModel which is exposed to QML. The implementation provides a Q_INVOKABLE longestCaption() method (can be found in kde-workspace/kwin/tabbox/clientmodel.(h|cpp)).

In kcmkwin/kwintabbox/layoutpreview.(h|cpp) an ExampleClientModel is provided which is also exposed to QML. This does not yet provide the Q_INVOKABLE method and thus creates runtime errors.

In this task the named method should be added to the ExampleClientModel with a similar semantic as the implementation in kwin/tabbox.

For this task a build-environment of kde-workspace on top of Qt5 and KF5 is required. If you don't have such a setup follow the steps of https://community.kde.org/Frameworks/Building - be aware that this is time consuming. Please only claim the task if you have this setup.