fix(extui): clear cmdline buffer for first message (#34490)

Problem:  Cmdline buffer is not cleared for a new message (since c973c7ae),
          resulting in an incorrect spill indicator. When the cmdline
          buffer is cleared, "msg_row" is not invalidated, resulting in
          an error. The extui module is untested.
          Return value of `vim.ui_attach()->callback` is undocumented.
Solution: Clear the cmdline buffer for the first message in an event
          loop iteration. Ensure msg_row passed as end_row does not
          exceed buffer length.
          Add `messages_spec2.lua` to test the extui module, keeping in
          mind that test coverage will greatly increase if this UI is made
          the default. As such, only tests for specific extui functionality
          unlikely to be covered by tests leveraging the current message grid.
          Document the return value of `vim.ui_attach()->callback`, it seems
          to make sense, and is also used to suppress remote UI events in
          `messages_spec2.lua`.

2 files changed, 17 insertions, 11 deletions

diff --git a/runtime/lua/vim/_extui/cmdline.lua b/runtime/lua/vim/_extui/cmdline.lua
index 4b9198d11d..53f14ccb4b 100644
--- a/runtime/lua/vim/_extui/cmdline.lua
+++ b/runtime/lua/vim/_extui/cmdline.lua
@@ -110,11 +110,11 @@ end
 
 --- Leaving the cmdline, restore 'cmdheight' and 'ruler'.
 ---
---@param level integer
+---@param level integer
 ---@param abort boolean
-function M.cmdline_hide(_, abort)
-  if M.row > 0 then
-    return -- No need to hide when still in cmdline_block.
+function M.cmdline_hide(level, abort)
+  if M.row > 0 or level > 1 then
+    return -- No need to hide when still in nested cmdline or cmdline_block.
   end
 
   fn.clearmatches(ext.wins.cmd) -- Clear matchparen highlights.
@@ -166,7 +166,7 @@ end
 --- Clear cmdline buffer and leave the cmdline.
 function M.cmdline_block_hide()
   M.row = 0
-  M.cmdline_hide(nil, true)
+  M.cmdline_hide(M.level, true)
 end
 
 return M
diff --git a/runtime/lua/vim/_extui/messages.lua b/runtime/lua/vim/_extui/messages.lua
index 1812b778dd..46a2b97db2 100644
--- a/runtime/lua/vim/_extui/messages.lua
+++ b/runtime/lua/vim/_extui/messages.lua
@@ -90,10 +90,12 @@ local function set_virttext(type)
   elseif #chunks > 0 then
     local tar = type == 'msg' and ext.cfg.msg.target or 'cmd'
     local win = ext.wins[tar]
-    local max = api.nvim_win_get_height(win)
-    local erow = tar == 'cmd' and M.cmd.msg_row or nil
-    local srow = tar == 'msg' and fn.line('w0', ext.wins.msg) - 1 or nil
-    local h = api.nvim_win_text_height(win, { start_row = srow, end_row = erow, max_height = max })
+    local erow = tar == 'cmd' and math.min(M.cmd.msg_row, api.nvim_buf_line_count(ext.bufs.cmd) - 1)
+    local h = api.nvim_win_text_height(win, {
+      max_height = api.nvim_win_get_height(win),
+      start_row = tar == 'msg' and fn.line('w0', ext.wins.msg) - 1 or nil,
+      end_row = erow or nil,
+    })
     local row = h.end_row ---@type integer
     local col = fn.virtcol2col(win, row + 1, h.end_vcol)
     local scol = fn.screenpos(win, row + 1, col).col ---@type integer
@@ -223,6 +225,11 @@ function M.show_msg(tar, content, replace_last, append, pager)
     cr = M[tar].count > 0 and msg:sub(1, 1) == '\r'
     restart = M[tar].count > 0 and (replace_last or dupe > 0)
     count = M[tar].count + ((restart or msg == '\n') and 0 or 1)
+
+    -- Ensure cmdline is clear when writing the first message.
+    if tar == 'cmd' and not will_pager and dupe == 0 and M.cmd.count == 0 then
+      api.nvim_buf_set_lines(ext.bufs.cmd, 0, -1, false, {})
+    end
   end
 
   -- Filter out empty newline messages. TODO: don't emit them.
@@ -367,9 +374,8 @@ function M.msg_show(kind, content, _, _, append)
       -- Store the time when an error message was emitted in order to not overwrite
       -- it with 'last' virt_text in the cmdline to give the user a chance to read it.
       M.cmd.last_emsg = kind == 'emsg' and os.time() or M.cmd.last_emsg
-      -- Should clear the search count now, which also affects the showcmd position.
+      -- Should clear the search count now, mark itself is cleared by invalidate.
       M.virt.last[M.virt.idx.search][1] = nil
-      M.msg_showcmd({})
     end
 
     -- Typed "inspection" messages should be routed to the pager.