How do I get lost windows back?

[hw]

On Wed, 2024-02-21 at 21:06 +0100, René J.V. Bertin wrote:
> On Wednesday February 21 2024 14:31:16 hw wrote:
> 
> > Unmapped?
> 
> One of the X11 ways to make a window disappear without destroying it:
> 
> https://linux.die.net/man/3/xunmapwindow

That seems to indicate that it can be used to make child windows
invisible and to generate BadWindow errors.

Is the konsole window always a child window, and why would it be
unmapped?  And how did it become visible again, and why did was it
moved outside the display?