If your window part does not have any parts that can scale both horizontally and vertically with the window part, then chain your parts together from top to bottom and left to right rather than from the outside in.

If the example window part did not have the list part and the multiline edit box, then you would get the best result by chaining all of the parts from the top down, leaving the bottom of the push buttons unattached. Then, if the window part grows larger, there is unused space, but the unused space is to the right of and below all the parts on the window part.

It is best to have unused space appear to the right of and below all of the parts on a window part rather than in the middle, as it would if you attached the push buttons to the bottom of the window part and the text parts and labels to the top of the window part.