> AFAICT from the Elisp Info docs, one is currently free to change the
 > parent-frame as one sees fit. That is, one can make a child a root
 > frame, a root frame a child, or change the parent of a child to another
 > root.
 >
 > I'm currently inclined to disallow parent-frame changes on ttys
 > entirely, unless it is used for something. Couldn't find something
 > today, but maybe something is a package outside Emacs' repo, or I'm just
 > ont looking for the right thing.

I'm not aware of people using multiple frames on TTYs and I have no idea
whether child frames out there are even intended to be reparented.

Reparenting is useful because you can set up one child frame for some
special purpose, make it invisible when you don't need it, and move it
to another frame and make it visible there whenever you want to.  On
GUIs, this approach has the advantage that you can avoid the (at least
here) costly process of setting up frame faces every time anew.  Emacs
tooltips do that by default and I recall that showing a tooltip every
time incurred two entire GC cycles here.

martin