The libvips generator passed 0 as the target height to vips_thumbnail,
so the value was rejected and dropped.
As a result, a preview of a 4000x5000 portrait image requested with e.g.
x=500&y=500 returned a ...x1920 image instead of a ...x1080 one, which
would be the next correct size in the pre-defined resolutions list. This
was due to the missing height, so it used the width (=1920) to determine
the longest side.
This also aligns it with the non-libvips behavior.
Jannik Stehle