Issue Summary

When using the Legacy Editor, we can use @ to mention users. When doing it on a longer page - the selector will not appear, and the autocomplete selector window is appearing at the bottom instead.

The issue is easily reproducible in Edge and Chrome.

Steps to Reproduce

1. Open Confluence.
2. Access a (longer) page in the Legacy Editor (seems to be anything more than a single window worth of data).
3. Try to tag someone.
4. The selector will not display
5. Scroll down to the bottom of the page and you will see the selector displaying beyond the publish and close options.

Expected Results

The autocomplete selector should appear where they are being inserted.

Actual Results

The selector window is displaying at the very bottom of the page.

Workaround

1. Users can convert their pages to the Cloud editor (when feasible).
2. If users scroll down to the end of the page, they should be able to find the selector there (please note that the external (browser) scrollbar should be used rather than the internal (editor) scrollbar, otherwise this second workaround won't work as the mention dropdown menu will get closed immediately).