Rich Text Editor - toolbar not sticky

I have quite an annoying problem with the Rich Text Editor -
when I scroll down (with having lots of text), the toolbar doesn’t stick to the top and instead it goes away.

I found some people with similar problems on older Umbraco versions. But no fix.

I use 8.4.0

Screenshot no.1

Screenshot no.2 (scrolled)


This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/100667-rich-text-editor-toolbar-not-sticky