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

chrome/browser/cocoa/tab_view.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.
 
 #include "chrome/browser/cocoa/tab_view.h"
 
 #include "chrome/browser/cocoa/nsimage_cache.h"
 #include "chrome/browser/cocoa/tab_controller.h"
 #include "chrome/browser/cocoa/tab_window_controller.h"
 
@@ skipping 249 matching lines @@
     NSPoint thisPoint = [NSEvent mouseLocation];
     CGFloat stretchiness = thisPoint.y - dragOrigin_.y;
     stretchiness = copysign(sqrtf(fabs(stretchiness))/sqrtf(kTearDistance),
                             stretchiness) / 2.0;
     CGFloat offset = thisPoint.x - dragOrigin_.x;
     if (fabsf(offset) > 100) stretchiness = 0;
     [sourceController_ insertPlaceholderForTab:self
                                          frame:NSOffsetRect(sourceTabFrame_,
                                                             offset, 0)
                                  yStretchiness:stretchiness];
-
+    // Check that we haven't pulled the tab too far to start a drag. This
+    // can include either pulling it too far down, or off the side of the tab
+    // strip that would cause it to no longer be fully visible.
+    BOOL stillVisible = [sourceController_ isTabFullyVisible:self];
     CGFloat tearForce = fabs(thisPoint.y - dragOrigin_.y);
-    if (tearForce > kTearDistance) {
+    if (tearForce > kTearDistance || !stillVisible) {
       draggingWithinTabStrip_ = NO;
       // When you finally leave the strip, we treat that as the origin.
       dragOrigin_.x = thisPoint.x;
     } else {
       return;
     }
   }
 
   NSPoint lastPoint =
     [[theEvent window] convertBaseToScreen:[theEvent locationInWindow]];
@@ skipping 366 matching lines @@
   [[NSGraphicsContext currentContext] restoreGraphicsState];
 }
 
 // Called when the user hits the right mouse button (or control-clicks) to
 // show a context menu.
 - (void)rightMouseDown:(NSEvent*)theEvent {
   [NSMenu popUpContextMenu:[self menu] withEvent:theEvent forView:self];
 }
 
 @end