Steps to replicate the issue:

• Visit https://doc.wikimedia.org/mediawiki-core/master/php/annotated.html
• Reduce viewport height (e.g. resizing browser window, or displaying Find bar on Firefox)

What happens?

A new global vertical scroll bar displays to see the footer which is now out of screen. This new scrollbar disturbs the use of the internal content scrollbar.
On Firefox, both scroll bars are even overlapping, making impossible to scroll the content using the scroll bar.

What should have happened instead?

Body height should be automatically reduced to fit with viewport height, so only the internal content scroll bar displays.
Workaround: refresh the page after having reduced the viewport height.

Software version:

I can’t reproduce at official Doxygen website, so this bug may have been fixed upstream.

Browsers:

Tested with Firefox 115 and Chromium 119, under LXQt and KDE environments.