Which wiki is this? I can see this on https://en.wikipedia.org/wiki/Main_Page but not https://en.wikipedia.org/wiki/Main_Page?safemode=1, https://www.wikidata.org/wiki/Wikidata:Main_Page and https://fr.wikipedia.org/wiki/Wikip%C3%A9dia:Accueil_principal so is this a gadget problem?

Do you have Twinkle installed? I did, and disabling that made the issue go away on English Wikipedia

@Jdlrobson This is Wikidata. It happens on all types of pages. Here are my scripts: https://www.wikidata.org/wiki/User:Lectrician1/common.js

@Lectrician1: Please follow https://www.mediawiki.org/wiki/Help:Locating_broken_scripts , try safemode, and report back. Thanks.

I did safemode and it still happens. It doesn't happen in Chrome for me. Likely a Firefox thing.

It looks like the background images that produce the gradient and a fake bottom border are extending beyond the bottom border as well as the side border dividers.

Removing margin-top: 1px additionally seems to fix it a bit:

However not completely with the background images re-enabled:

I'm still unable to replicate this. Note, the gradients have been removed this week with the train deployment. Is it possible that fixed this issue?

Yeah I don't see any issue with the new headers.