I must have pressed something because LR is no longer behaving as it used to.
In the Develop module my zoom level (I believe) was set to Fit and if I switched from a lanscape orientated photo to a portrait one, I'd always be able to see the whole photo. If I then zoomed in or out, I could return to that zoom to fit level just by using Command+/-
Now, however, there doesn't seem to be a zoom to fit option and if I have it set to 50% or 53% which is fine for landscape orientated photos, I then only see part of a portrait orientated photo.
How do I reset it so that it just automatically shows me the whole photo as large as possible whether it's portrait or landscape?
It was a bit of a mixture of the Fit/Fill in Navigator (hadn't noticed until now that it had moved there), pressing Z to toggle the zoom mode and manuall trying to zoom to the right size each time to compensate for not having Fit enabled.
If it says Fit in the Navigator, it should work fine on Lightroom Classic 10.3 on my Mac, as shown below. What I usually do is first press the Z key, the shortcut for View > Toggle Zoom View, which toggles between the current magnification and Fit.
if I have it set to 50% or 53% which is fine for landscape orientated photos, I then only see part of a portrait orientated photo.
That part sounds normal. Because images are different sizes, a fixed percentage always risks that the next image might not exactly fit, even if it was the same orientation. Fit view always makes it fit.
There have been some recent subtle changes to how zooming works, but being able to use Fit magnification is still possible.