All popup windows inside of PRs (Reviewers, Assignees, Labels, etc...) get hidden behind the content in the main area. Have confirmed issue does not happen when Refined Github is disabled. This also only happens on Github Enterprise, does not happen on github.com

Also, in case it is relevant, we are running GitHub Enterprise Server 2.17.5
If it helps, also seeing this on GitHub Enterprise 2.15.3, with refined-github 19.9.17
Tried disabling a number of individual features today in hopes of narrowing it down, but haven't had any luck.
Please let me know if there's anything I can do to help find the cause of or fix for this 🙂
I don't use Enterprise so it's hard to see what's wrong. Perhaps it's sticky-discussion-sidebar but this issue can only be fixed if a GHE user sends a PR.
Oh, it is sticky-discussion-sidebar 🙂
If you send a PR I can test it in GH and GHE, otherwise it'll be a few
weeks before I can look at it.
On Wed, Sep 18, 2019, 4:29 AM Phil McArdle notifications@github.com wrote:
Oh, it is sticky-discussion-sidebar 🙂
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I can have a look at this after work today :)
Hi, maybe this helps! It looks like .discussion-sidebar is missing position: relative for z-index to take effect. Once I add that, z-index is working:

https://<ghe url>/app/app/assets/stylesheets/components/discussion-timeline.scss
I've confirmed that @kamranayub solution does fix it. As a workaround, you can add this as Custom CSS in the add-on preferences:
.discussion-sidebar {
position: relative;
}
Can confirm that adding @jvtrigueros's CSS to the plugin custom CSS resolves the issue.
Does sticky-discussion-sidebar work on GHE? Does disabling the feature fix the issue?
discussion-sidebar is the old class name (removed in https://github.com/sindresorhus/refined-github/pull/2314)
@fregante No, sticky-discussion-sidebar does not work on GHE. Can confirm that disabling it also fixes the issue.
Most helpful comment
I've confirmed that @kamranayub solution does fix it. As a workaround, you can add this as Custom CSS in the add-on preferences: