refactor(channel): defer hidden console allocation to :detach

Refactor #37977: instead of allocating a hidden console at startup, borrow the parent's console via AttachConsole() and only create an isolated hidden console when :detach is called, with fd 0/1/2 re-bound to the new CONIN$/CONOUT$.

src/nvim/channel.c

@@ -38,6 +38,7 @@
 #include "nvim/msgpack_rpc/channel.h"
 #include "nvim/msgpack_rpc/server.h"
 #include "nvim/os/fs.h"
+#include "nvim/os/os.h"
 #include "nvim/os/os_defs.h"
 #include "nvim/os/shell.h"
 #include "nvim/terminal.h"
@@ -549,13 +550,22 @@ uint64_t channel_from_stdio(bool rpc, CallbackReader on_output, const char **err
     os_set_cloexec(stdin_dup_fd);
     stdout_dup_fd = os_dup(STDOUT_FILENO);
     os_set_cloexec(stdout_dup_fd);
-
-    // The server may have no console (spawned with UV_PROCESS_DETACHED for
-    // :detach support). Allocate a hidden one so CONIN$/CONOUT$ and ConPTY
-    // (:terminal) work.
+    const bool restart_alloc_console = os_env_exists("__NVIM_RESTART_ALLOC_CONSOLE", true);
+    if (restart_alloc_console) {
+      os_unsetenv("__NVIM_RESTART_ALLOC_CONSOLE");
+    }
     if (!GetConsoleWindow()) {
-      AllocConsole();
-      ShowWindow(GetConsoleWindow(), SW_HIDE);
+      if (restart_alloc_console) {
+        AllocConsole();
+        ShowWindow(GetConsoleWindow(), SW_HIDE);
+      } else if (!AttachConsole(ATTACH_PARENT_PROCESS)) {
+        // Borrow the parent's console so CONOUT$ resolves to the real terminal,
+        // preserving io.stdout rendering (e.g. SIXEL/Kitty images).  Only fall
+        // back to a hidden AllocConsole when there is no parent console (e.g.
+        // launched from a non-console parent).
+        AllocConsole();
+        ShowWindow(GetConsoleWindow(), SW_HIDE);
+      }
     }
     os_replace_stdin_to_conin();
     os_replace_stdout_and_stderr_to_conout();

src/nvim/ex_docmd.c

@@ -17,6 +17,7 @@
 #include "nvim/api/private/dispatch.h"
 #include "nvim/api/private/helpers.h"
 #include "nvim/api/ui.h"
+#include "nvim/api/vim.h"
 #include "nvim/api/vimscript.h"
 #include "nvim/arglist.h"
 #include "nvim/ascii_defs.h"
@@ -89,6 +90,9 @@
 #include "nvim/os/input.h"
 #include "nvim/os/os.h"
 #include "nvim/os/os_defs.h"
+#ifdef MSWIN
+# include "nvim/os/os_win_console.h"
+#endif
 #include "nvim/os/shell.h"
 #include "nvim/path.h"
 #include "nvim/plines.h"
@@ -5020,6 +5024,13 @@ static void ex_restart(exarg_T *eap)
     server_stopped = server_stop(listen_arg, true);
   }
 
+#ifdef MSWIN
+  bool restart_alloc_console_env = false;
+  if (os_setenv("__NVIM_RESTART_ALLOC_CONSOLE", "1", 1) == 0) {
+    restart_alloc_console_env = true;
+  }
+#endif
+
   CallbackReader on_err = CALLBACK_READER_INIT;
   // This temporary bootstrap channel is closed intentionally once we obtain
   // the new server address. Don't forward child stderr to the current UI.
@@ -5031,6 +5042,11 @@ static void ex_restart(exarg_T *eap)
                                        CALLBACK_READER_INIT, on_err, CALLBACK_NONE,
                                        false, true, true, detach, kChannelStdinPipe,
                                        NULL, 0, 0, NULL, &exit_status);
+#ifdef MSWIN
+  if (restart_alloc_console_env) {
+    os_unsetenv("__NVIM_RESTART_ALLOC_CONSOLE");
+  }
+#endif
   if (!channel) {
     emsg("cannot create a channel job");
     goto fail_1;
@@ -5907,7 +5923,10 @@ static void ex_detach(exarg_T *eap)
       emsg(e_invchan);
       return;
     }
-    chan->detach = true;  // Prevent self-exit on channel-close.
+    // Prevent self-exit on channel-close.
+    Error detach_err = ERROR_INIT;
+    nvim__chan_set_detach(chan->id, true, &detach_err);
+    api_clear_error(&detach_err);
 
     // Server-side UI detach. Doesn't close the channel.
     Error err2 = ERROR_INIT;
@@ -5928,6 +5947,12 @@ static void ex_detach(exarg_T *eap)
     // XXX: Can't do this, channel_decref() is async...
     // assert(!find_channel(chan->id));
 
+#ifdef MSWIN
+    // After UI/channel detach, move this server off the parent's console so it
+    // survives terminal closure and still has working CONIN$/CONOUT$.
+    os_swap_to_hidden_console();
+#endif
+
     ILOG("detach current_ui=%" PRId64, chan->id);
   }
 }

src/nvim/os/os_win_console.c

@@ -56,6 +56,19 @@ void os_replace_stdout_and_stderr_to_conout(void)
   assert(conerr_fd == STDERR_FILENO);
 }
 
+/// Detach from the current console and switch stdio to a hidden private one.
+///
+/// Used when an embedded server must outlive its parent console, while keeping
+/// CONIN$/CONOUT$ and ConPTY functional for :terminal and stdio writes.
+void os_swap_to_hidden_console(void)
+{
+  FreeConsole();
+  AllocConsole();
+  ShowWindow(GetConsoleWindow(), SW_HIDE);
+  os_replace_stdin_to_conin();
+  os_replace_stdout_and_stderr_to_conout();
+}
+
 /// Resets Windows console icon if we got an original one on startup.
 void os_icon_reset(void)
 {