Autocomplete selector window not appearing in the Legacy Editor

Issue Summary

When using the Legacy Editor, we can use shortcuts such as // for inserting dates, @ to mention users, [ for links, and ! for image/media, and { for macros.

When trying to use those shortcuts at the bottom of a longer page - the selector will not appear.

The issue is easily reproducible in Edge and Chrome. 

Steps to Reproduce

1. Open Confluence.
2. Create a page in the Legacy Editor
3. Add some content to it, making it a longer page (seems to be anything more than a single window worth of data)
4. Try to tag someone, insert date, insert link, add image/media, or add a macro to the bottom of the page.
5. The selector will not display
6. Scroll down to the bottom of the page and you may see the selector displaying beyond the publish and close options

Expected Results

The selectors should appear where they are being inserted.

Actual Results

The selector window is displaying at the very bottom of the page or in some circumstances not at all.

Workarounds

1. Users can convert their pages to the Fabric editor.
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).