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WordPress 4.9 included a significant refactoring of the WYSIWYG editor when displayed on the front end, so that when inserting links into the editor it would have a more modern inline UI.
This appears to work well with FSE themes (like twentytwentyfive), but has broken the link button functionality in standard PHP themes (like twentytwentyone, Astra, etc.). When using the Visual tab scrolling may be hijacked, and when using the Text tab the “pop-up” no longer appears in a modal and is shown unstyled at the end of the page.
The issue has been raised with our developers who will work on a solution soon.
You may need to use a plugin like Core Rollback to revert WordPress to a 4.8.x version in the meantime.
