bug#28611: 26.0.60; iconifying a child frame seems to be problematic

[martin rudalics]

> Steps to reproduce:
>
>    1. $ emacs -Q &
>    2. Enlarge the frame a bit by mouse.
>    3. Type (make-frame `((parent-frame . ,(selected-frame)))) C-j
>    4. Click the newly created child frame.
>    5. Type (iconify-frame) C-j
>
> Result:
>
>    The child frame becomes unresponsive.
>    The parent frame can take input, but the cursor is still hollow.
>
> Maybe an attempt to iconify a child frame should cause an error.

We warn about this in the Elisp manual:

   * The semantics of maximizing and iconifying child frames is highly
     window-system dependent.  As a rule, applications should never
     invoke these operations for child frames.

I'm not sure whether we should do more about this given the fact that
these operations may and should work well on other window systems.

BTW: Does ‘make-frame-visible’ invoked for the child frame make it
responsive again?

martin