Problem: vim.on_key() called for each message while cmdline is open.
Cursor is on a seemingly random column when pager is entered.
Entering the pager while the cmdline is expanded can be more
convenient than pressing "g<".
Pager window is unnecessarily clamped to half the shell height.
Setting 'laststatus' while pager is open does not adjust its
dimensions.
Solution: Only call vim.on_key() once when dialog window is opened.
Ensure cursor is at the start of the first message when
entering the pager.
Enter the pager window when "<CR>" is pressed while the
cmdline is expanded.
Don't clamp the pager window height.
Set message windows dimensions when 'laststatus' changes.
