CodeMirror sometimes loads whitespace at the end of the inputbox in the source editor, which in turn causes the cursor to select text many lines above where is intended. This can be fixed by typing/deleting text anywhere in the inputbox.
This only seems to happen occasionally so it's difficult to reproduce. It did happen a few minutes ago for me in Safari Version 12.0 (14606.1.36.1.9) on MacOS 10.14. I can't actually replicate this for the task :) but perhaps others can.
I'm not sure I understand the question; I was just using the source editor, I'm not sure if it has a particular name. This is on enwiki, so whichever one enwiki uses. :)
@jrbs: See https://www.mediawiki.org/wiki/Editor for the long list of editors provided on Wikimedia websites.
Thanks @jrbs. This bug is related to several others that have already been reported and unfortunately the problem is with the CodeMirror library itself. We're expecting an upgrade to that library in the near future. Hopefully that will help.
I don't know if this is still valid, but I'm safely assuming it's not an issue in CM6.
CodeMirror 5 has officially been retired, thus closing as invalid. Please re-open or file a new bug should the issue persist with the new version.