what to do to make windows non-resizable?

[wysota]

Maybe because the designer doesn't know the actual (runtime) contents of the widgets and this would mix things up ( could give you a wrong preview of the dialog ).

Size constraint is an attribute of a layout, not of the widget. I think it would be nice to be able to have such a property in designer just like we have margin and spacing available, but let's face it - how often do you set a size constraint on a layout?

You can always break the layout and set another one.

You can't set a new layout for a main window.

If that is the case, then it should use its child layout to manage its size, no?

No. Layout manages children, not parents. Period. What would happen if you'd wrap a widget with a layout and children into another layout? Which layout should be responsible for managing the size? Besides, you have to have something "fixed" at some point, otherwise you couldn't use size policies as any widget with "expanding" size policy would cause the top level widget to expand to screen width/height.

A QMainWindow never has a parent widget,

Why not? I used a QMainWindow as a child widget a few times...

but is in need of a size-controller. Ergo it should use the first child size-controller it finds.

The window manager is its controller as it is a child of the root window of your screen.

What is the use of the sizePolicy property of the QMainWindow, seeing as setting it to fixed does not give us anything?

Designer uses a generic way of managing properties. If you wanted to hide properties which were unusable for top-level widgets, you'd have to have a way of marking any property as such and there would be a problem if you wanted to place one form inside another as the widget which was designed as a top-level widget might in the end be a child of some other widget.

Finally, I presume you did the corrections you did using a text editor and not Designer, right?

You should never modify Designer generated files by hand.

[jamadagni]

You should never modify Designer generated files by hand.

Why never? I realize that it can be labeled not recommended as it is easy to muck up such a complicated XML file, but I have found it a quick way to make tiny tweaks and some cleanups that Designer won't do for me.

[wysota]

Why never?

Because of two reasons.
1. All changes will be overwritten next time you save the file from within Designer,
2. You have to create a real widget class anyway, so there is no point in modifying any of the generated files as you can do the same elsewhere without pushing yourself into trouble.

Ok, three reasons...
3. It's very prone to errors.

I realize that it can be labeled not recommended

If you're willing to say that aiming a gun at your head and pressing the trigger is not recommended as well, then I can agree with what you said.

but I have found it a quick way to make tiny tweaks and some cleanups that Designer won't do for me.

What kind of tweaks? I don't know if you're referring to modifying xml files saved by Designer or files generated by uic. I guess the former and I really can't see any possible reason to do that...