Plain-GitHub and `sticky-sidebar` lead to zindex problem #4419

When adding an emoji at the right site of the textarea box, the GitHub created emoji popup does not play nice with images and dividers in the sidebar, see screenshot.

I did try this out without RefinedGitHub and also saw the problem when editing a PR descriptions, so I guess this is not Refined-GitHub related.

But this addon could fix it in regards of sticky-sidebar as the error can occure more often with this addon on.