This long standing bug was caused by `Pane`'s focus_in handler bouncing
the focus to another handle.
Because focus resolution happens _after_ a frame has been rendered, the
only way to deal with this case is to schedule another frame to be
redrawn. However, we where suppressing all window refreshes that occur
during a focus transfer, causing this focus change to be completely
missed. However, changing this behavior can lead to infinite notify
loops, due to drawing a frame causing another to be rendered.

This PR fixes this problem narrowly by adding an `on_next_frame()`
callback in the pane's focus handle, so that the focus changes take
effect almost immediately. But only for this case, where we know it
doesn't cause infinite notify loops.

TODO:
- [x] Fix the infinite notify loop bug or determine a third way to fix
this lag

Release Notes:

- Fixed a bug where shifting focus to the terminal panel could be slow