Problem: Messages routed to the pager to be shown in full, enter the
pager automatically, yielding another "press-q-prompt".
Solution: Only enter the pager when requested explicitly. Otherwise,
close the pager on the next typed mapping, unless that mapping
entered the pager.
Problem: "cmdline_show" event may be emitted with an invalid cursor
position, causing a redraw that will clear the match highlight.
Solution: Mark the cursor position as valid so that a "cmdline_show"
callback that updates the screen does not clear the match highlight.
Problem: OptionSet autocmd emitted with invalid grids after entering
tabpage with different 'cmdheight'.
Solution: First call `tabpage_check_windows()` before changing 'cmdheight'.
Add a test that exercises the `vim._extui` cmdline.
