This allows the relationship to be restored when the window becomes eligible
again.

winemac: When a window becomes ineligible to be a parent, make the relationship latent in its children, too.

It may be necessary to reorder to some extent because the clicked window is
behind a sibling at the same level, but that shouldn't move it later in the
list than higher-level siblings.

Cocoa gets buggy if the list of child windows isn't in z-order.