Thanks for the link. Now I see what is happening.

However a few coments on the topic. Whilst everybody is speaking about this line:
Qt Code: 
  1. QMetaObject::connectSlotsByName(this);
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actually this line is just nowhere in the project files. Can be pretty confusing that it appears in the ui_mainwindow.h because this header file does not even exist before compiling. So assuming we have only these files:
main.cpp
mainwindow.cpp
mainwindow.h
mainwindow.ui
myproject.pro

The mentioned line can not be found in these files. The tricky thing is that when generating ui_mainwindow.h the critical line is _always_added - no matter if I created any connection with the above described method or not. So I can also declare them by hand with the proper syntax and the magical "right click - go to slot - clicked()" mechanism does the very same and adds no additional clue to the code.