Our edit cards use position:fixed, and a recent change to Minerva added a stacking context to .content, which triggers a known issue in Chrome that breaks the position:fixed.
|mediawiki/skins/MinervaNeue||master||+4 -2||[fix] [Visual Editor] [LESS] [content] create stacking context differently|
To clarify, the problem wasn't with a stacking context, it was with using transform: …; to create it. The fix is to instead use position: relative; z-index: …;.
I think using the latter is a good practice in general – while this is probably not a problem for Minerva, applying transforms also disables subpixel text rendering on Windows (making it appear blurry), and in general transforms are buggier than one would expect (in this case it was a bug in Chrome, but here's another recent example in Firefox: T224984).