Scroll bar issue | Follow-up from "Feature/merge panels"

In the Tree View and the annotation panel there are some issues with the scroll bar. Behavior observed so far:

multiple scroll bars for a single panel (next to each other)
no scroll bar at all (so content not reachable)
scroll bar present, but very bottom not reachable (like if we have height: 100vh what cannot work in a panel below a header

Recommendation is to solve this for all panels or containers inside a panel by a set of CSS rules. Needs documentation how to use flexbox correctly in this case.

The following discussion from !139 (merged) should be addressed:

@mrodzis started a discussion: (+2 comments)

I have two scrollbars now.

If this is the intended behavior, I'll approve.