- Avoid shared state. Pass `focus` to set_pos()/expand_msg() instead of
a shared `pager_focus` flag: the flag is only cleared when set_pos()
actually enters the pager, so ":messages" from inside the pager left
it set.
- pager_shown(): the pager window is invalid after leaving it with "q".
- Reuse pager_shown() in expand_msg().