fix: implement django-cms/djangocms-text-ckeditor#568

[wesleyboar]

Overview

Implement the Issue #568 fix from PR #590 until we update our ckeditor to v5.

This will save H.P. (main CMS admin/designer) so many tiny bits of time, because she must resize a modal every time she wants to edit source, which is many times everyday.

Related

N/A

Changes

• extend djangocms-text-ckeditor css
• duplicate the relevant fix

Testing

1. Be logged into CMS as admin.
2. Add a Text plugin instance.
3. Edit Text plugin instance content as source (the "[<>] Source" button, top-right).
4. Confirm the textarea (in which to edit source markup) fills available vertical space.

UI

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