The vertical alignment, from 0 (start) to 1 (end).
The yalign property determines the vertical alignment of the children
inside the squeezer’s size allocation. Compare this to
GtkWidget:valign, which determines how the squeezer’s size
allocation is positioned in the space available for the squeezer.
This will affect the position of children too tall to fit in the squeezer as they are fading out.