For vertical page previews the margin between the bottom of the image and the first line of text is inconsistent, potentially based on whether the page preview opens above or below the linked text. Here is a screenshot of me hovering over the same link, once where it's towards the top of the screen and once where it's towards the bottom:
=== Design notes
The desired amount of padding is 25px
=== Developer notes
I tried to debug this a little bit but the inspector was acting a little funky with the `$($0).trigger('mouseenter');` trick. It kept opening the page preview above the link even if the link was super close to the top of the screen. We recently worked on T192928 and T192773. Not sure if it's related to either of those patches.