May be a simple fix with some CSS.

.dropdown-menu {
    max-height: 280px;
    overflow-y: auto;
}

FWIW on my mobile device your long dropdown does have a scroll bar.

It's a a little hard to test these changes with the pkgdown website. Could you install this branch and rebuild your site locally?

pak::pak("r-lib/pkgdown@issue-2712")

IMO this already looks ok, and the real solution is to add some hierarchy to your articles.

It's a a little hard to test these changes with the pkgdown website. Could you install this branch and rebuild your site locally?

pak::pak("r-lib/pkgdown@issue-2712")

It doesn't seem to have added a scroll bar. Was the .dropdown-menu class used for the articles dropdown? It seems like it was only defined.

IMO this already looks ok, and the real solution is to add some hierarchy to your articles.

Hierarchy = combining related articles? In hindsight, it probably would have been better to organize the articles by manuscript figure. Unfortunately, with the manuscript being published, I do not currently have the time or resources to spend on making major changes to the package (especially since no one else appears to be using it...). I will at least ensure that future R packages like it will not repeat the same mistakes.