The image below shows the effect when applied to the find files dialog. Rectangles have been drawn around all of the boxes. Blue boxes indicate horizontal boxes. You can see that the entire window is also a horizontal box. Red boxes indicate vertical boxes.
Zigzags show where there are flexible elements.
In this case, the spring is flexible so a zigzag appears above it. Lines show where inflexible elements are, in this case the text, the input field and the buttons. White square boxes indicate the edges of elements. In the image, you might notice a number of additional boxes in place. This is because each XUL element is made up of a number of boxes itself.