Auto-expanding layout with Qt-Designer

I'm using the Qt Designer.

I want to create a QVBoxLayout which will automatically expand to fill the whole window.

The layout of the QVBoxLayout remains fixed.

How can I cause the QVBoxLayout to expand and fill the entire window through the designer?

After creating your QVBoxLayout in Qt Designer, right-click on the background of your widget/dialog/window (not the QVBoxLayout, but the parent widget) and select Lay Out -> Lay Out in a Grid from the bottom of the context-menu. The QVBoxLayout should now stretch to fit the window and will resize automatically when the entire window is resized.

According to the documentation, there needs to be a top level layout set.

A top level layout is necessary to ensure that your widgets will resize correctly when its window is resized. To check if you have set a top level layout, preview your widget and attempt to resize the window by dragging the size grip.

You can set one by clearing the selection and right clicking on the form itself and choosing one of the layouts available in the context menu.

I've tried to find a "fit to screen" property but there is no such.

But setting widget's "maximumSize" to a "some big number" ( like 2000 x 2000 ) will automatically fit the widget to the parent widget space.