What is the desired behaviour of 'Fit image to ...'

[guest]

>> "Fit image to window heignt":
Intuitively i would expect that the height of an image is enlarged (or shrinked)
to fit in the defined viewer windows height.

And then, the width of the windows is calculated proportionally, that means,
to be large enough (or small enough) that the image exactly may fit herein.
(Additionally the window should be (by option) centered vertically)

- Actually the image is truncated in width if the width increases the window's width.

>> "Fit image to desktop, all"
Expectation same as above, with the difference that the height of the viewer window
is increased to the desktop's height first.

- Actually only the window increases to the desktop, but the image stays at is is,
and so we see a big gray frame around it.

The behaviour desired is imo more intuitive (and its at least the way acdsee works),
but maybe i misunderstood something ?

What is the correct behaviour?

Kind regards