To ensure that the location bar will not reach the minimum width.

chrome/browser/ui/cocoa/toolbar/toolbar_controller.mm

 // Copyright (c) 2012 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/ui/cocoa/toolbar/toolbar_controller.h"
 
 #include <algorithm>
 
 #include "base/mac/bundle_locations.h"
 #include "base/mac/mac_util.h"
@@ skipping 71 matching lines @@
 @interface ToolbarController()
 @property(assign, nonatomic) Browser* browser;
 - (void)addAccessibilityDescriptions;
 - (void)initCommandStatus:(CommandUpdater*)commands;
 - (void)prefChanged:(const std::string&)prefName;
 - (BackgroundGradientView*)backgroundGradientView;
 - (void)toolbarFrameChanged;
 - (void)pinLocationBarToLeftOfBrowserActionsContainerAndAnimate:(BOOL)animate;
 - (void)maintainMinimumLocationBarWidth;
 - (void)adjustBrowserActionsContainerForNewWindow:(NSNotification*)notification;
+- (void)browserActionsContainerWillDrag:(NSNotification*)notification;
 - (void)browserActionsContainerDragged:(NSNotification*)notification;
 - (void)browserActionsContainerDragFinished:(NSNotification*)notification;
 - (void)browserActionsVisibilityChanged:(NSNotification*)notification;
 - (void)adjustLocationSizeBy:(CGFloat)dX animate:(BOOL)animate;
 - (void)updateWrenchButtonSeverity:(WrenchIconPainter::Severity)severity
                            animate:(BOOL)animate;
 @end
 
 namespace ToolbarControllerInternal {
 
@@ skipping 478 matching lines @@
   }
 }
 
 - (void)createBrowserActionButtons {
   if (!browserActionsController_.get()) {
     browserActionsController_.reset([[BrowserActionsController alloc]
             initWithBrowser:browser_
               containerView:browserActionsContainerView_]);
     [[NSNotificationCenter defaultCenter]
         addObserver:self
+           selector:@selector(browserActionsContainerWillDrag:)
+               name:kBrowseActionContainerWillTranslateOnXNotification

[Yoyo Zhou, 2014/10/27 23:45:26]

typo: kBrowserAction...

[Malcolm, 2014/10/28 06:56:09]

Oops, I lost the 'r'.
On 2014/10/27 23:45:26, Yoyo Zhou wrote:

+             object:browserActionsController_];
+    [[NSNotificationCenter defaultCenter]
+        addObserver:self
            selector:@selector(browserActionsContainerDragged:)
                name:kBrowserActionGrippyDraggingNotification
              object:browserActionsController_];
     [[NSNotificationCenter defaultCenter]
         addObserver:self
            selector:@selector(browserActionsContainerDragFinished:)
                name:kBrowserActionGrippyDragFinishedNotification
              object:browserActionsController_];
     [[NSNotificationCenter defaultCenter]
         addObserver:self
@@ skipping 28 matching lines @@
   [browserActionsContainerView_ setGrippyPinned:locationBarAtMinSize_];
   [self adjustLocationSizeBy:
       [browserActionsContainerView_ resizeDeltaX] animate:NO];
 }
 
 - (void)browserActionsContainerDragFinished:(NSNotification*)notification {
   [browserActionsController_ resizeContainerAndAnimate:YES];
   [self pinLocationBarToLeftOfBrowserActionsContainerAndAnimate:YES];
 }
 
+- (void)browserActionsContainerWillDrag:(NSNotification*)notification {
+  CGFloat deltaX = [[notification.userInfo objectForKey:kTranslationWithDelta]
+      floatValue];
+  CGFloat locationBarWidth = NSWidth([locationBar_ frame]);

[Yoyo Zhou, 2014/10/27 23:45:26]

I'm not very familiar with this code, so please bear with me. Is there some
reason we can't use maintainMinimumLocationBarWidth?

[Malcolm, 2014/10/28 06:56:09]

The -maintainMinimumLocationBarWidth only adjusts the location bar to guarantee
the minimum size of it, not the frame of browserActionsContainerView. So if we
just call -maintainMinimumLocationBarWidth, the browserActionsContainerView will
still overlap the locationbar, although the locationbar will be minimum size.
Actually, the frame of browserActionsContainerView is changed in the internal of
it (-mouseDragged:). By setting the property "canDragLeft" here can tell the
browserActionsContainerView that whether the dragging is allowed in
-mouseDragged:. If not, the -setFrame will not executed in -mouseDragged: of
browserActionsContainerView.
On 2014/10/27 23:45:26, Yoyo Zhou wrote:

+  BOOL locationBarWillBeAtMinSize =
+      (locationBarWidth + deltaX) <= kMinimumLocationBarWidth;
+
+  // Prevent the |browserActionsContainerView_| from dragging if the width of
+  // location bar will reach the minimum.
+  [browserActionsContainerView_ setCanDragLeft:!locationBarWillBeAtMinSize];
+}
+
 - (void)browserActionsVisibilityChanged:(NSNotification*)notification {
   [self pinLocationBarToLeftOfBrowserActionsContainerAndAnimate:NO];
 }
 
 - (void)pinLocationBarToLeftOfBrowserActionsContainerAndAnimate:(BOOL)animate {
   CGFloat locationBarXPos = NSMaxX([locationBar_ frame]);
   CGFloat leftDistance;
 
   if ([browserActionsContainerView_ isHidden]) {
     CGFloat edgeXPos = [wrenchButton_ frame].origin.x;
@@ skipping 178 matching lines @@
 - (void)hideDropURLsIndicatorInView:(NSView*)view {
   // Do nothing.
 }
 
 // (URLDropTargetController protocol)
 - (BOOL)isUnsupportedDropData:(id<NSDraggingInfo>)info {
   return drag_util::IsUnsupportedDropData(profile_, info);
 }
 
 @end