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`.

1 files changed, 7 insertions, 3 deletions

diff --git a/runtime/lua/vim/_extui.lua b/runtime/lua/vim/_extui.lua
index d27d665c15..1b9d81b091 100644
--- a/runtime/lua/vim/_extui.lua
+++ b/runtime/lua/vim/_extui.lua
@@ -40,9 +40,6 @@ local M = {}
 
 local function ui_callback(event, ...)
   local handler = ext.msg[event] or ext.cmd[event]
-  if not handler then
-    return
-  end
   ext.tab_check_wins()
   handler(...)
   api.nvim__redraw({
@@ -59,6 +56,9 @@ function M.enable(opts)
   if opts.msg then
     vim.validate('opts.msg.pos', opts.msg.pos, 'nil', true, 'nil: "pos" moved to opts.target')
     vim.validate('opts.msg.box', opts.msg.box, 'nil', true, 'nil: "timeout" moved to opts.msg')
+    vim.validate('opts.msg.target', opts.msg.target, function(tar)
+      return tar == 'cmd' or tar == 'msg'
+    end, "'cmd'|'msg'")
   end
   ext.cfg = vim.tbl_deep_extend('keep', opts, ext.cfg)
 
@@ -76,11 +76,15 @@ function M.enable(opts)
   end
 
   vim.ui_attach(ext.ns, { ext_messages = true, set_cmdheight = false }, function(event, ...)
+    if not (ext.msg[event] or ext.cmd[event]) then
+      return
+    end
     if vim.in_fast_event() then
       scheduled_ui_callback(event, ...)
     else
       ui_callback(event, ...)
     end
+    return true
   end)
 
   -- Use MsgArea and hide search highlighting in the cmdline window.