Problem: Resize events during startup may clear an active external
cmdline, which is then not redrawn.
UI2 VimResized autocommand does not work.
UI2 message appearance may be altered by inherited window
options. The message separator uses the wrong fillchar.
Solution: Unset cmdline_was_last_redrawn when clearing the screen, such
that cmdline_show is re-emitted.
Ensure set_pos function is called without arguments.
Ensure such options are unset. Use 'fillchars'->msgsep.
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.
