"Open with" PR dropdown overlapping `sticky-conversation-sidebar` #4490

On any PR, such as https://github.com/sindresorhus/refined-github/pull/4395.

This is not an easy issue because it's due to z-index. A solution must be found by looking at the previous commits that affected the z-index of the sidebar and avoid regressions.

Sticky

<img width="467" alt="Screen Shot 19" src="https://user-images.githubusercontent.com/1402241/122523001-c1a17380-d040-11eb-867c-0588cc48b6ec.png">

Not sticky

<img width="476" alt="Screen Shot 20" src="https://user-images.githubusercontent.com/1402241/122522997-c0704680-d040-11eb-9cb9-2ab156359004.png">