1. Click "show desktop" applet button, (or press CTRL-ALT-d)
2. All currently open windows become "hidden"
3. Open a new window

At this point, metacity implicitly disables the "show desktop" mode. This is
indicated visually in that the "show desktop" applet toggles from pressed to
unpressed.

Additionally, there is a change in the state of all windows that were hidden.
A hidden window can be restored to visible by an explicit disabling of the
show desktop mode. However, after the implicit disable mode, no window can
remain in a hidden state, (since there is no longer an explicit way to restore
the windows).

So the question remains, what state should these hidden windows be given
when implicitly disabling "show desktop". Two obvious choices are "visible"
and "minimized".

According to the relevant bug history:

http://bugzilla.gnome.org/show_bug.cgi?id=92335

transitioning to visible was extremely annoying in many cases, (I agree---an
implicit operation should not cause such drastic visual changes).

The resolution of 92335 was a decision to transition the windows to minimized
state. However, this is not currently happening, (as can be seen in the
workspace switcher and the window selector).

See also bug 162010 which I'm filing concurrently.