Thanks for answering.

You most likely posted the answer in your first line.
Is your system working correctly?
Yes, it is. But may be it's not properly configured - I am working on it. Except this trouble everything works fine. It's a some kind of terminal - I don't need there Gnome or KDE. I set up auto-login, and when system boot - X server starts and run my program.


Do you mean that the mainwindow remains the parent of the input dialog? Then yes, that's by design and how you wrote your code.
No, sorry for my bad English.
I meant that the window which QApplication:: focusWidget () returned is the dialog parent (mainwindow). I expected that the QApplication:: focusWidget () will return a pointer to QInputDialog (because it is modal, and as my mind must have focus).
Mainwindow set as central widget in qApp. Keyboard works fine in any other place of program.
As I can see system uses policy "focus-follow-mouse" by default (I do not know where it is configured), but even when mouse over QInputDialog - it also doesn't react on keyboard.