Having tested this, it seems to work, but unfortunately does still reintroduce #4652. The reason for this is (I think) that the DOM element that does the scrolling when it gets too thin has changed from being public-DraftEditor-content to public-DraftEditor-root (as per your PR).

In order to fix this, you may also need to edit this line: matrix-org/matrix-react-sdk@7e0fecb#diff-fa2e6c7a24d3ee8c284aa720b8f399edR466 such that the editor root is scrolled to the bottom instead of the editor content.

Thanks @lukebarnard1,
it seems like this is the problem. Changed your highlighted line(s) to:

            // Scroll to the bottom of the editor if the cursor is on the last line of the
            // composer. For some reason the editor won't scroll automatically if we paste
            // blocks of text in or insert newlines.
            if (textContent.slice(selection.start).indexOf("\n") === -1) {
                this.refs.editor.refs.editor.parentNode.parentNode.scrollTop = this.refs.editor.refs.editor.parentNode.parentNode.scrollHeight;
            }

And it seems to work. Will do another pull request against matrix-react-sdk for that. Not sure what to do with the long line, maybe needs some refactor.
matrix-react-sdk PR: matrix-org/matrix-react-sdk#1682

(updated comment to correct PR ref)