It’s very typical for users to upload images which are screenshots taken on mobile.
Currently the recent changes prevent these mobile screenshots from displaying in a reasonable way.
Cropping to the top of the image is undesirable, unexpected and considered “broken” by users.
Please also consider adding a visual indicator on the smaller image when it has been cropped.
I noticed this earlier today and just pushed a fix 
https://github.com/discourse/discourse/commit/ebd4b4577168644ff1132af85215bfa2034ecdbb
I’m open for suggestions
Maybe the crop font-awesome icon?
I’m just unsure if you know at the time the lightbox runs that it was cropped versus resized…
Perhaps something like this which is always displayed on a cropped image:
Please also note that for extremely tall images clicking the image and then showing full-screen is a poor solution…
… mainly because you are unable to scroll and you are still limited by the height of the users screen.
Note the result of clicking the image in the first post of the feature request:
If you click on the image once it’s full-screen, the image will be “real size” and you’ll be able to scroll
