@Volker_E noticed that the sidebar.template renders the current page title with

<h1 class="firstHeading" id="firstHeading">{{titletext}}</h1>

it should use a different id, they're meant to be unique.

See also T107738: Blueprint doesn't indicate current page in its left nav links if the page doesn't have a TOC.. Note the current page's title also appears in the main content area (with the same CSS), and may be one of the pages in the sidebar navigation.