You solve this by using a layout control (a UI control specifically for managing the size/position of its child controls). For example using a visual form builder you could drag-n-drop a "vertical layout control" and then drag-n-drop controls inside it to be resized/positioned in a vertical stack.

The window itself can have a layout control that stretches to fill the window regardless of resizing. Layout changes would cascade down through the tree of UI controls and everything would resize/reposition accordingly.