Separators in navigation tree dropdown

Steps to reproduce:

• On the main page, open the tree navigation dropdown menu by clicking on the arrow right of the "Home" text
• Expand two adjacent sections, like "Sandbox" and "XWiki"

Expected result:

The two sections are expanded without any separator between them.

Actual result:

A separator is displayed between the two expanded (sub-)trees. This can also be experienced by expanding adjacent subtrees:

The separator has been introduced by a style change in XWIKI-22485.