Start a drag if the tab is dragged off the sides of the window....

chrome/browser/cocoa/tab_window_controller.mm

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "chrome/browser/cocoa/tab_window_controller.h"
 
 #include "base/logging.h"
 #import "chrome/browser/cocoa/tab_strip_view.h"
 
 @interface TabWindowController(PRIVATE)
@@ skipping 137 matching lines @@
 - (void)insertPlaceholderForTab:(TabView*)tab
                           frame:(NSRect)frame
                   yStretchiness:(CGFloat)yStretchiness {
   [self showNewTabButton:NO];
 }
 
 - (void)removePlaceholder {
   [self showNewTabButton:YES];
 }
 
+- (BOOL)isTabFullyVisible:(TabView*)tab {
+  // Subclasses should implement this, but it's not necessary.
+  return YES;
+}
+
 - (void)showNewTabButton:(BOOL)show {
   // subclass must implement
   NOTIMPLEMENTED();
 }
 
 - (void)detachTabView:(NSView*)view {
   // subclass must implement
   NOTIMPLEMENTED();
 }
 
@@ skipping 20 matching lines @@
 }
 
 - (void)setTab:(NSView*)tabView isDraggable:(BOOL)draggable {
   if (draggable)
     [lockedTabs_ removeObject:tabView];
   else
     [lockedTabs_ addObject:tabView];
 }
 
 @end