People often create custom widgets with container widgets (QWidget/QFrame...) and forget to make the isContainer() method return true where appropriate. This results in bugs like the parent task (cannot break layout, etc).

PLease add a comment explaining the issue and prompting people to change it to the template file there.

The perfect solution would be to ask Designer's widget database about the container-ness of the parent class and make the method return this value, however, this is apparently hard to do.