The way that comparison refresh disables and reenables textview
sensitivity and handlers, it destroys pane focus and stops us from
updating our cursor data (e.g., what are the next and previous chunks).

This can lead to a situation where our comparison results have changed,
but we haven't updated our cursor details. Because next/previous actions
(among others) don't need a currently focused pane, these will try to
run with stale data and break.

The fix here, while it seems odd, is to refocus the last focused pane
(if there is one) on comparison refresh. The focus change causes the
cursor structure to re-update, and the cursor data used by actions is no
longer stale.