I am surprised that I can't seem to google this, but:
How can I have GIMP fit the image to the window when it opens it?
Prefs -> Image Windows -> Initial zoom ration: "Show entire image" is
selected, but neither shows the entire image nor fits it to the window.
My camera images open at 25% zoom, which results cropping off the top
and bottom of the window, and requiring a ctrl-shift-J on every opened
image to be able to see it all.
(This is with 2.9.5, but it's been this way as long as I recall.)