[WIP] Fix comment popup display issue on mobile devices

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>Comment popup is not displayed correctly on some mobile devices</issue_title>
<issue_description>Popup on my iPad (2019), Safari mobile, recent version of ep_comments_page, etherpad 1.8.12:

</issue_description>

Comments on the Issue (you are @copilot in this section)

@JohnMcLear @seballot any idea how this can happen?