in GNU Emacs OSX, how to hide title bar?

You can set the variable ns-auto-hide-menu-barto a non-nil value to hide the menu bar.

Also, you can position the window title above the top of the screen, if you do it programmatically. (OS X doesn't allow you to drag the frame above the top of the screen, though.)

For example: The following will position the editable area at the top of the display.

(setq ns-auto-hide-menu-bar t)
(set-frame-position nil 0 -24)
(tool-bar-mode 0)
(set-frame-size nil 150 80)     ;; Pick values matching your screen.

Note: This might require Emacs 24.4 (which is still in pretest).

Alternatively, you could use the package Multicolumn to position and resize the frame (it's not OS X specific, but it's aware of features like auto-hiding the menu bar.)

EDIT: I've contributed this patch as --with-no-titlebar for the homebrew formula "emacs-plus".

---

Below is my update of an old patch by Jay McCarthy. The following patch is compatible with GNU Emacs 25.2:

diff --git a/lisp/cus-start.el b/lisp/cus-start.el
index d9ad0a5..9e52d0f 100644
--- a/lisp/cus-start.el
+++ b/lisp/cus-start.el
@@ -446,6 +446,7 @@ minibuffer-prompt-properties--setter
        (ns-use-native-fullscreen ns boolean "24.4")
              (ns-use-fullscreen-animation ns boolean "25.1")
              (ns-use-srgb-colorspace ns boolean "24.4")
+             (ns-use-titled-windows ns boolean "25.2")
        ;; process.c
        (delete-exited-processes processes-basics boolean)
        ;; syntax.c
diff --git a/src/nsterm.m b/src/nsterm.m
index 1b44a73..d013101 100644
--- a/src/nsterm.m
+++ b/src/nsterm.m
@@ -6775,11 +6775,13 @@ - (BOOL)isOpaque
    maximizing_resize = NO;
  #endif

-  win = [[EmacsWindow alloc]
+  win = [[EmacsFSWindow alloc]
              initWithContentRect: r
                        styleMask: (NSResizableWindowMask |
  #if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_7
-                                  NSTitledWindowMask |
+                                  (ns_use_titled_windows ?
+                                   NSTitledWindowMask :
+                                   NSWindowStyleMaskBorderless) |
  #endif
                                    NSMiniaturizableWindowMask |
                                    NSClosableWindowMask)
@@ -6812,6 +6814,7 @@ - (BOOL)isOpaque
    [win setTitle: name];

    /* toolbar support */
+  if ( ns_use_titled_windows ) {
    toolbar = [[EmacsToolbar alloc] initForView: self withIdentifier:
                          [NSString stringWithFormat: @"Emacs Frame %d",
                                    ns_window_num]];
@@ -6833,6 +6836,7 @@ This avoids an extra clear and redraw (flicker) at frame creation.  */
    }
  #endif
    FRAME_TOOLBAR_HEIGHT (f) = 0;
+  }

    tem = f->icon_name;
    if (!NILP (tem))
@@ -8759,6 +8763,12 @@ Nil means use fullscreen the old (< 10.7) way.  The old way works better with
  This variable is ignored on OSX < 10.7 and GNUstep.  */);
    ns_use_srgb_colorspace = YES;

+  DEFVAR_BOOL ("ns-use-titled-windows", ns_use_titled_windows,
+     doc: /*Non-nil means to include a title on windows.  Nil means to
+omit the title on OSX >= 10.7.  This variable is ignored on OSX <
+10.7.  Default is nil.  */);
+  ns_use_titled_windows = NO;
+
    /* TODO: move to common code */
    DEFVAR_LISP ("x-toolkit-scroll-bars", Vx_toolkit_scroll_bars,
          doc: /* Which toolkit scroll bars Emacs uses, if any.

Many people call this "borderless", sometimes without referring to the title bar at all.