The thumbnails are 120x68 and cropped with object-fit: cover, so any text on a
slide is unreadable and the edges are cut off. Picking the right slide to
reorder or delete meant opening images by hand to tell them apart.
Hovering a thumbnail now shows the whole slide, bounded by the viewport rather
than the image so a 3300x2550 upload does not fill the screen, with the filename
underneath.
Fixed position rather than inside the row: the list scrolls and a
relatively-positioned parent would clip it. pointer-events: none so the preview
can never sit between the cursor and the move or delete buttons.
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