When using the desktop improvements skin on the [[:ro:Metrou]] page, a double scrollbar is shown. This happens when the content is shorter than the left menu and the menu is longer than the screen (reproduced here).
Using FF77 on Win 10, 1920x1080 resolution.
I can confirm with Firefox 81 on Linux; for a short moment a second scrollbar is shown, then it's not shown anymore.
@Strainu: Does it remain visible for you?
Yes, the second scrollbar remains visible when I open the main page of German Wikipedia in Firefox 81.0.1 on macOS 10.13.6.
New Vector by design currently has a min-width
https://gerrit.wikimedia.org/g/mediawiki/skins/Vector/+/64d03c85ac0123775cb7b4b4975b59cfafc7d00b/resources/skins.vector.styles/layouts/screen.less#44
A double scroll bar will appear below this threshold, as we can't guarantee the design to still work at that resolution.
As the project continues we aim to eventually remove that (perhaps as part of T276566), but this is working exactly as intended for now.