[Backport] Usability: Site Map item search input gets hidden constantly by scroll bar

If a scroll bar is present on sitemap items in the Experience Manager, the search input gets hidden all the time when using it.

Reproduction

• Go to Experience Manager, e.g. https://cms.demo.onehippo.com/experience-manager/hippogogreen-preview
• Open the sitemap
• Make the browser window small enough so a scroll bar appears for the sitemap
• Click search input
  expected --> input stays visible
  actual --> a scroll takes place, hiding the input
• Scroll to make the input visible again
• Type 'a'
  expected --> input stays visible
  actual --> a scroll takes place, hiding the input