Make New-sidebar scrolling behavior like issue sidebar

Open username-removed-1661688 requested to merge snookerkevin/gitlab-ce:master into master

What does this MR do?

It improves the scrolling to behave like the issue sidebar. See merge request below. It essentially does the same changes as the merge request below.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10831

Why was this MR needed?

Scrolling not nice in iOS

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

@annabeldunstone @iamphill UX frontend

Edited by username-removed-1661688