Problem: 'showcmd' causes flickering when pressing "g<" to enter the
pager when the cmdline is expanded for messages.
Initial keypress for an incomplete mapping is not giving 'showcmd'
feedback while cmdline is expanded for messages (which is only
dismissed upon the vim.on_key callback after 'timeoutlen').
Solution: Delay dismissing expanded cmdline when vim.on_key() callback
receives "g".
Place 'showcmd' "last" virtual text during expanded cmdline.