I am very aware of Notepad++, but it does not have the colour coding by keystroke of text that WP5.1+ has, document switching by keystroke, nor does it have UNDELETE (as distinct from 'Undo'). So I would still like a solution to the WP problem that the insert marker appears not to be movable my means of mouse cursor.
But since you mentioned Notepad++, I will raise an issue, at the risk of annoying those who insist in the single-thread character of forums. I use Notepad++ to search for the first (and last) characters that I want to KEEP, in the source code of emails that I want to 'clean up' before forwarding them. I would rather do that searching in the source code view of the actual email, instead of copying all the source code first into Notepad++, and then trying to locate in the email source code, the key found in the COPY of the source code. So if you know a way of performing 'CTRL F' in the source code of emails, I'd be delirious with joy.