Steps to replicate the issue:

• Go to https://en.wikipedia.org/wiki/Albert_Einstein or any other page that links to the article "Wave-particle duality"
• Hover over the link to "Wave-particle duality"

What happens?:
A vertical fade to white box is included in the preview partway through the last line.

What should have happened instead?:
There should be a horizontal fade at the end of the line as normal

Other information (browser name/version, screenshots, etc.):
This is due to the fact that "Wave-particle duality" has a block quote immediately following the text which is included in the preview. This fade is in the ::after element of the blockquote html element. Found using Google Chrome and Vector legacy skin. Likely related to https://phabricator.wikimedia.org/T213107 which added the fade to block quotes which were previously cut off.