feat(extui): route to "more" window if it is open (#34190)

Problem:  An already open "more" window is the most convenient place
          to route messages to with `cfg.msg.pos == 'cmd'`. Instead we
          route to the cmdline (unless that message also exceeds
          'cmdheight').
Solution: Route to "more" window while it is open and scroll to bottom
          to show the newest message. This is more convenient and more
          efficient due to not writing to the target buffer first (which
          is done to calculate the height of the message).
          (Ensure message highlights are deleted when text is replaced.)

runtime/lua/vim/_extui/messages.lua

@@ -213,7 +213,7 @@ function M.show_msg(tar, content, replace_last, append, more)
     -- Split at newline and write to start of line after carriage return.
     for str in (chunk[2] .. '\0'):gmatch('.-[\n\r%z]') do
       local repl, pat = str:sub(1, -2), str:sub(-1)
-      local end_col = col + #repl
+      local end_col = col + #repl ---@type integer
 
       if line_count < row + 1 then
         api.nvim_buf_set_lines(ext.bufs[tar], row, -1, false, { repl })
@@ -226,8 +226,12 @@ function M.show_msg(tar, content, replace_last, append, more)
       width = tar == 'box' and math.max(width, api.nvim_strwidth(curline)) or 0
 
       if chunk[3] > 0 then
-        local opts = { end_col = end_col, hl_group = chunk[3] } ---@type vim.api.keyset.set_extmark
+        api.nvim_buf_set_extmark(ext.bufs[tar], ext.ns, row, col, {
-        api.nvim_buf_set_extmark(ext.bufs[tar], ext.ns, row, col, opts)
+          end_col = end_col,
+          hl_group = chunk[3],
+          undo_restore = false,
+          invalidate = true,
+        })
       end
 
       if pat == '\n' then
@@ -321,10 +325,14 @@ function M.msg_show(kind, content, _, _, append)
   elseif kind == 'return_prompt' then
     -- Bypass hit enter prompt.
     vim.api.nvim_feedkeys(vim.keycode('<CR>'), 'n', false)
-  elseif kind == 'verbose' and append_more == 0 then
+  elseif kind == 'verbose' then
     -- Verbose messages are sent too often to be meaningful in the cmdline:
     -- always route to box regardless of cfg.msg.pos.
     M.show_msg('box', content, false, append)
+  elseif append_more > 0 and ext.cfg.msg.pos == 'cmd' then
+    -- Append message to already open 'more' window.
+    M.msg_history_show({ { 'spill', content } })
+    api.nvim_command('norm! G')
   elseif ext.cmd.prompt then
     -- Route to prompt that stays open so long as the cmdline prompt is active.
     api.nvim_buf_set_lines(ext.bufs.prompt, 0, -1, false, { '' })
@@ -333,12 +341,8 @@ function M.msg_show(kind, content, _, _, append)
   else
     -- Set the entered search command in the cmdline (if available).
     local tar = kind == 'search_cmd' and 'cmd' or ext.cfg.msg.pos
-    if tar == 'cmd' and ext.cmdheight == 0 then
-      return
-    end
-
     if tar == 'cmd' then
-      if ext.cmd.level > 0 and ext.cmd.row == 0 then
+      if ext.cmdheight == 0 or (ext.cmd.level > 0 and ext.cmd.row == 0) then
         return -- Do not overwrite an active cmdline unless in block mode.
       end
       -- Store the time when an error message was emitted in order to not overwrite
@@ -398,10 +402,10 @@ function M.msg_history_show(entries)
   end
 
   -- Appending messages while 'more' window is open.
-  append_more = entries[1][1] == 'spill' and append_more + 1 or 0
+  if append_more == 0 then
-  if append_more < 2 then
     api.nvim_buf_set_lines(ext.bufs.more, 0, -1, false, {})
   end
+  append_more = append_more + 1
 
   for i, entry in ipairs(entries) do
     M.show_msg('more', entry[2], i == 1 and append_more < 2, false)