fix(messages): proper multiline Lua print() messages #31205

Problem:  Separate message emitted for each newline present in Lua
          print() arguments.
Solution: Make msg_multiline() handle NUL bytes. Refactor print() to use
          msg_multiline(). Refactor vim.print() to use print().

2 files changed, 5 insertions, 9 deletions

diff --git a/runtime/doc/ui.txt b/runtime/doc/ui.txt
index 4ee8121034..4e8253c47a 100644
--- a/runtime/doc/ui.txt
+++ b/runtime/doc/ui.txt
@@ -796,6 +796,7 @@ must handle.
 		"echomsg"	|:echomsg| message
 		"echoerr"	|:echoerr| message
 		"lua_error"	Error in |:lua| code
+		"lua_print"	|print()| from |:lua| code
 		"rpc_error"	Error response from |rpcrequest()|
 		"return_prompt"	|press-enter| prompt after a multiple messages
 		"quickfix"	Quickfix navigation message
diff --git a/runtime/lua/vim/_editor.lua b/runtime/lua/vim/_editor.lua
index b4a1e0fc15..44f17b3f85 100644
--- a/runtime/lua/vim/_editor.lua
+++ b/runtime/lua/vim/_editor.lua
@@ -1151,21 +1151,16 @@ end
 --- @param ... any
 --- @return any # given arguments.
 function vim.print(...)
-  if vim.in_fast_event() then
-    print(...)
-    return ...
-  end
-
+  local msg = {}
   for i = 1, select('#', ...) do
     local o = select(i, ...)
     if type(o) == 'string' then
-      vim.api.nvim_out_write(o)
+      table.insert(msg, o)
     else
-      vim.api.nvim_out_write(vim.inspect(o, { newline = '\n', indent = '  ' }))
+      table.insert(msg, vim.inspect(o, { newline = '\n', indent = '  ' }))
     end
-    vim.api.nvim_out_write('\n')
   end
-
+  print(table.concat(msg, '\n'))
   return ...
 end