Set the thumbnail height to auto instead of 154px so that there's not a
big empty gap beneath thumbnails that aren't 150px high.
This requires setting dimensions on the <img> tag itself so that the
image height is known before loading, otherwise the tooltip will be
mispositioned after the image loads in.
We set the min-width and min-height instead of the <img> width and
height properties because our calculated dimensions are sometimes
off-by-one compared to the actual dimensions. I'm not sure how libvips
calculates the thumbnail dimensions, but this avoids forcing a slightly
wrong aspect ratio, which produces ugly resizing artifacts.
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