[Back-port to 4.0] Side-panel tabs disappear when pages are scrolled

The tabs in the left side-panel of the channel manager ('Sitemap' and - in edit mode - 'Components') disappear when the list of pages or components is scrolled down. The tabs should not scroll with the page, but remain fixed and visible at the top of the list.

Steps to reproduce:

1. Open a channel (e.g. the UK Website on https://cms.demo.onehippo.com).
2. Open the left side-panel. Make sure a vertical scrollbar is visible. If not, decrease the height of your browser.
3. Scroll down in the list of pages.

Expected: the tab 'Sitemap' remains visible
Actual the tab 'Sitemap' scrolls off-screen