files: text editor bug: menububble "add link" vertical location KO in long MD files.

[berteh]

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Steps to reproduce

1. Create a looong MD file in Nextcloud "files" app (or upload and open attached MD file Looong test.md.txt)
2. open/edit online and scroll to the bottom
3. select some text portion at the bottom of it

Expected behaviour

The menububble "add link" dialog should popup next to the text currently selected.

Actual behaviour

The menububble "add link" dialog appears at the top of the document, outside current visible zone.

Server configuration

Web server:
https://nextcloud05.webo.cloud/

Nextcloud version: (see Nextcloud admin page)
production

Client configuration

Browser:
Chromium 87.0.4280.88 (Official Build)

Operating system:
Ubuntu 20.04 (64-bit)

Logs

Browser log

Browser log

see attachement, no errors, only deprecation warnings.
browser-1612361329332.log

[mejo-]

This should have been fixed by #1903.