Layouts caused me a lot of confusion when I first came across them using the designer.

If you use this to build your UIs you will see a layouts section where you can drag and drop a layout onto your UI.

Do not use this, it will make the UI form the container of your layout just like any other item dragged and dropped on to it, which is probably not what you want and is unlikely to behave as you expect. By default there is nothing to resize the container, so the layout will never get resized.

Instead right click on the item whose layout is to be managed, choose layout from the menu and attach the required layout this way. You can control-click to choose multiple items.

If you are using a container, eg a Tab Widget, you will need to place its contents into layouts first (vertical and horizontal seem to work best) and then allocate the container itself to a another layout.

I hope this makes sense. Good luck.