bug#53745: [External] : bug#53745: [DOC] About invisible frames

[Drew Adams <drew.adams@oracle.com>]

> Could you provide some use cases for invisible frames?

Whatever you can imagine for why you might want
to hide a frame ;-) - there's no limit.

To start with, invisibility gives frames a state
with two possible values, distinguishing them -
for whatever purpose.

And one of those hides the frame.  You can easily
and quickly split the set of frames into two sets,
any time, any way you like, for any purpose.

Most Emacs frame functions let you optionally act
only on the visible frames.  Making some invisible
temporarily means you can exclude them from such
actions - only the visible frames are affected;
the invisible ones are not.

This lets you act in ways similar to what Dired's
`t' (`dired-toggle-marks') does - 2 sets of frames
to work with: in front of, and behind, the looking
glass.  You can omit (hide, make invisible) the
marked or the unmarked files.  (Similarly, with
Bookmark+ you can show just the marked or just
the unmarked bookmarks.)

You can quickly flip visibility for any or all
frames.  You can, for example, hide all but one,
to unclutter - and later restore all.

Invisible, as opposed to iconified, frames don't
show up as icons (e.g. in the MS Windows Task Bar,
or on a desktop).  You see as icons only what you
want to see as icons, even though the invisible
frames are still present etc.