We only need the observer once to focus the left column on page load.
After that, we don't need to be listening to DOM changes.

Without disconnecting the observer, this broke '@'-name completion to
reference other users. Because that completion adds a new element (the
user list picker) to the column, our mutation observer gets called, and
we focus the left column, un-focussing the comment area.

Glad to have finally discovered why '@'-name completion in Jira comments
broke for me.

This one didn't work out because the element isn't on the page even at
'document-end'.