Further clarification:

More important details from MediaWiki original issue report:
I recently spotted a visual problem with the horizontal gradient (which is used to show the article content continues) at the end of the article description text: its large height causes it to cover part of the previous line.

After checking the HTML code, I found the problem. The text line height is set to 20px and the gradient pseudo-element has the height 24px. The pseudo-element should be the height of a line of text (20px here). To prevent the issue happening again if the text font size changes, the height should be set to "1em".

Please fix this minor issue - an image showing my screenshots of the problem [shown below] is available at "Problem demo image shown below" and any article with a tall letter such as "g" on the end of the second last line of the Page Previews/Hovercards text should cause this problem.

Using rems is pretty safe given popups is JS only (ie10+) and it is supported in ie9+ (when not used in font, use it in font-size instead).

Change 356464 had a related patch set uploaded (by Bmansurov; owner: Bmansurov):
[mediawiki/extensions/Popups@master] Reduce height of horizontal gradient

https://gerrit.wikimedia.org/r/356464

Change 356464 merged by jenkins-bot:
[mediawiki/extensions/Popups@master] Reduce height of horizontal gradient

https://gerrit.wikimedia.org/r/356464

@ABorbaWMF: @bmansurov's change should've been deployed to the Beta Cluster. You'll need to find some articles with long previews so you see spot the gradient.

@ABorbaWMF - adding test articles to task description

Looks good on Beta. Tried the example article. Also, the 'fox' link on the Dog article works well.