Steps to replicate the issue (include links if applicable):

• Go to https://en.wikipedia.org/wiki/Nauru
• Open developer tools
• Add the following CSS: body.vector-sticky-header-visible { --color-base: red; }
• Scroll down so the sticky header becomes visible, to see the CSS take effect
• Make the viewport width smaller than the desktop cutoff (1120px)
• Scroll up and down from the top again

What happens?:

• The class is added and removed from body even when the header isn't actually visible (because it is hidden with CSS at lower viewports)

What should have happened instead?:

• The class should stay true to its name, and only be present when the sticky header is actually visible (whether or not that is due to viewport size)

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):