fix(messages): reset redirection message column at message start #38068

Problem:  Leading message newlines (not emitted with ext_messages since
          4260f73) were responsible for resetting the redirection message
          column (while the newline itself is later pruned...).
Solution: Ensure the redirection column is reset at the start of a message.
          (Instead of re-adjusting all the newline callsites which can
          themselves hopefully be pruned if ext_messages is enabled by
          default.)

src/nvim/message.c

@@ -1632,6 +1632,7 @@ void msgmore(int n)
   }
 }
 
+static int redir_col = 0;  // Message column used in redir_write().
 void msg_ext_set_kind(const char *msg_kind)
 {
   // Don't change the label of an existing batch:
@@ -1641,6 +1642,12 @@ void msg_ext_set_kind(const char *msg_kind)
   // need refactoring the msg_ interface to not be "please pretend nvim is
   // a terminal for a moment"
   msg_ext_kind = msg_kind;
+
+  // Need to reset the redirection column at the start of a message, for which
+  // leading newlines are responsible without kUIMessages. Unrelated to setting
+  // the kind but this is called more consistently at the start of a message
+  // than msg_start() at this point.
+  redir_col = msg_ext_append ? redir_col : 0;
 }
 
 /// Prepare for outputting characters in the command line.
@@ -3371,7 +3378,6 @@ void msg_check(void)
 static void redir_write(const char *const str, const ptrdiff_t maxlen)
 {
   const char *s = str;
-  static int cur_col = 0;
 
   if (maxlen == 0) {
     return;
@@ -3390,7 +3396,7 @@ static void redir_write(const char *const str, const ptrdiff_t maxlen)
   if (redirecting()) {
     // If the string doesn't start with CR or NL, go to msg_col
     if (*s != '\n' && *s != '\r') {
-      while (cur_col < msg_col) {
+      while (redir_col < msg_col) {
         if (capture_ga) {
           ga_concat_len(capture_ga, " ", 1);
         }
@@ -3404,7 +3410,7 @@ static void redir_write(const char *const str, const ptrdiff_t maxlen)
         if (verbose_fd != NULL) {
           fputs(" ", verbose_fd);
         }
-        cur_col++;
+        redir_col++;
       }
     }
 
@@ -3431,17 +3437,17 @@ static void redir_write(const char *const str, const ptrdiff_t maxlen)
         putc(*s, verbose_fd);
       }
       if (*s == '\r' || *s == '\n') {
-        cur_col = 0;
+        redir_col = 0;
       } else if (*s == '\t') {
-        cur_col += (8 - cur_col % 8);
+        redir_col += (8 - redir_col % 8);
       } else {
-        cur_col++;
+        redir_col++;
       }
       s++;
     }
 
     if (msg_silent != 0) {      // should update msg_col
-      msg_col = cur_col;
+      msg_col = redir_col;
     }
   }
 }

test/functional/api/vim_spec.lua

@@ -439,6 +439,12 @@ describe('API', function()
       assert_alive()
       eq(false, exec_lua('return _G.success'))
     end)
+
+    it('redir_write() message column is reset with ext_messages', function()
+      exec_lua('vim.ui_attach(1, { ext_messages = true }, function() end)')
+      api.nvim_exec2('hi VisualNC', { output = true })
+      eq('VisualNC       xxx cleared', api.nvim_exec2('hi VisualNC', { output = true }).output)
+    end)
   end)
 
   describe('nvim_command', function()